I know this is a bit like how long is a piece of string but i have heard that puppies are generally half their adult size and weight at 20 weeks. I have a cocker who is 14 weeks today and is already 6kgs!! An average adult cocker is only 13.5kgs so am a little worried im going to end up with a new breed of giant cocker!!!

I think that is likely to vary hugely by breed and individually.

I've never heard that before.

I've had litter mates that are different sizes at 4 months but ended up the same as adults so I can't see how it works.

I'd say that there was no 'generally' about it... my huge lad Cody was the smallest in his litter.. now he's the biggest but at 20 weeks we wasn't noticeably bigger... some dogs grow early and some keep growing steadily...

I don't buy that at all.  Willow at 18 weeks is at least 10kg (probably 11 by now) and 16" at the shoulder, and there aren't many 32" tall collies around!