Possible Rack Design and Birthing bin Questions

[PigsnPythons]

I think that most people recommend each female having her own birthing bin, typically how big is that bin?

Also, I'm considering building a rat rack that would have two different sized bins, ex.

_____BIG_____
_LIL__ __LIL__
_LIL__ __LIL__
_____BIG_____

So that I can have the little bins as birthing bins and the big bins as breeding, growup, and female rest bins. This way I can fit my rotation into a shorter rack (I'm 5'2" so a 6+ tall rack is going to be an issue). Does anyone see any possible issues with this? Assuming I can find them the smaller bins would be roughly half the size of a medium concrete mixing tub (I'm going to have the big tubs slide out lengthwise instead of width wise ....so the width of the little bin would be roughly half the length of the bigger bin).

Also, is a birthing bin of that size going to be too small during the last week or week and a half of weaning? Would it be an option to move females that have litters that are 2+ weeks old to the same big bin? Assuming no more than two females per bin? Or do you still lose babies even at that age?

Also, if you are planning on replacing a retiring breeder with her daughter, from which litter do you typically try to hold back a female?

Thanks!!