Generally does the smallest of the litter become a small adult, and the largest of the litter remain the largest - or do the others catch up?  Is there any disadvantage in choosing the smallest of the litter?  

In answer to the first part of your question - no :)

Really, it varies - but I have found that, generally speaking, it is impossible to predict the adult size based on the size of an eight week old puppy, and sometimes the smallest in the litter surpass their siblings and the biggest turn out not to have been as big as you thought they might be.  Sometimes the smallest stay small - but not always.

Not in my experience.

No neither I would say. In one litter, my heaviest born(a bitch) ended up the smallest as an adult.
My most recent litter the heaviest bitch is the smallest now & lightest bitch is the biggest now.

Depends of the reason for them being small at birth, if it's genetic then it isn't likely to change, if it is poor nourishment due to poorly functioning placenta/very large litter then they are likely to catch up when they are getting their fair share of food.

Neither of those applied to my litters, as it wasnt genetic or poor nourishment!!!!!!!

I don't think 5 is a large litter - There is quite a significant difference between the largest and smallest - They are only  2 weeks old now so a bit early to tell anything.  I was just fancying the markings on the smallest one - but depends on how he is.  I am not seeing them in the flesh until nearly 4 weeks old and even then I think it is too early to choose one.  I do have first choice (after the owners one she is keeping) but someone is travelling an awful long way to see them a few days later so I have to express some interest in one!  Don't know what is normal about choosing puppies.

Five may not be a big litter in some breeds and very big in others, pups who are undernourished at birth are scrawny/scraggy with no healthy layer of flesh over their bones, pups who are genetically small are in perfect proportion and plump/chubby. Simply having too many squashed into one horn could compromise growth and development.

At 2 weeks it is too early to pick a pup and certainly not from pics and markings are not top priority when making a pick, in the past I have had to change my mind re colour/pattern because that pup didn't come up to the breed standard. What are you wanting the pup for? Some experts can pick pups with potential at birth but things can go wrong before they reach 8 weeks which is the ideal age for evaluation and pick.

If you are wanting a pup for conformation shows/breeding then a breeder shouldn't be forcing you to pick at 4 weeks as that is too early and surprised the breeder can make her pick by then too although being her pups she could change her mind later.

I would go and see the pups at 4 weeks, eliminate the definite No's but ask if you can delay making the final pick between say 2 pups till later.

Cody, my 25" border collie was the smallest puppy in a litter of 4 (2 boys and 2 girls) at 7 weeks, although clearly is the largest adult ;-)  he wasn't squashed (4 is small for a collie litter) and he was certainly not undernourished... he was the first pup born and very well fed & looked after, simply the smallest but grew more :-)

I did have a big litter of GSDs of which 9 were born alive and 2 dead... the last live birth (dead pups were no 1 and no 8) was much smaller than the others, clearly he'd been shoved up in a corner.  However, by 7 weeks he was the equal of the smaller other pups and grew just as big as the other boys :-)  He was a very bright pup with bags of personality.

I am looking for good temperament more than anything else although of course I would like a dog that pleases the eye.  These are collies and all the pups are very healthy looking.  Agility is my hobby right now - and both the parents are good agility dogs.  However I know that is not the most important thing either - because a dog is only as good as it's owner!  

Yes Penny - I am looking at a collie litter for agility - the biggest one must have masked 2 of them in the scan only 2 days before they were born.  They were expected on the sunday but arrived on friday night.  I dreamed all that night of helping deliver pups.  In my dream there were only 2 - one that was white with spots on and the other huge one that was all white.  I can remember being horrified that the big white one would be deaf - and I couldn't have that one.  On the following monday I met another club member for the first time with - an all white deaf collie - never met one before.  So funny dreams!  One of the pups has an all white head with a spot over the eye - some sort of premonition??!!  She was the only bitch in the litter and is staying with her owner.   I was very relieved that there were 4 others to choose from and that the choice of bitch or dog was made for me. 

An all white head would concern me re sight and hearing in a dog that normally has patches of colour on their head.

My last puppy was for Agility (now 9 months old) not a Collie though a Mali. When we picked him we picked him because of his character and confidence and he was just the 'right one' for us. He was a big chunky pup though (they all were similar in size apart from one slightly smaller one) and from day 1 I had said I wanted a more slight and smaller dog (like our mali bitch). Everyone has joked that he was going to be huge and at every meeting with his litter mates he has always been the biggest up until about 6 months. Now they seem to have caught up and he hasn't grown a huge amount lately, seems to be slowing down so for a boy he actually isn't that big although he is still chunky. I know he hasn't finished growing yet and his body shape will change but I cant see him getting to the size of my older boy so I don't think puppy size does matter all that much.

That was my fear I think coming out in my dream.  Mum has a fair amount of white - not excessive - on her head - the smallest one has less white on his head which is the look I prefer - like the Dad who is very handsome.  Rather biased I am afraid because of how my present dog Meg looks too. 

I have found that puppy size is no indication of potential adult size.

I purchased a girl from fantastic lines, who as a puppy was the second largest in the litter. At maturity she is now the smallest.

Her daughter, who was half the size of her littermates, at seven months is already bigger than her mom.

One of my other girls was the smallest in her litter as a puppy and at maturity is now huge....

Thank you everyone for your replies - if the smallest pup looks like being for me then I won't have any reservations about size. 

If the dog is for working (ie agility) can you not just sit in the room with them and see which one shows the most interest in you?  If a dog chooses you it is likely to work better for you.  Well, that´s my opinion and experience anyway although no doubt not everyone would agree with me.