
I had a six ounce pup in my first klitter where the others were 12, 13 and 14, she was about a pound behind in weight by 8 weeks, but by 10 months was a perfectly normal average size.
If it is a big litter make sure you plug that one in more often when the others are asleep, as it won't be able to take as much in one go as the others. this will be especially true once she starts getting out of the wheloing box to lie beside it, afterthe first week. When I have had a wee one, I used to put it on a piece of vetbed by the bitch for extra nursing seesions!