Congrats on the new additions! Fry are so fun to watch, when you can find them. I just wanted to add onto what Brendan said about the floating breeding traps. They will slow the growth of the fry a bit so it will take longer to get the fry to the size that they won't be getting eaten. I used the breeder traps for a long time until I did an experiment. I kept 3 fry in the trap and put 3 fry in the main tank, made sure that there were plenty of hiding places in the main tank, and within about a month's time, the fry in the main tank were about twice the size of the ones in the breeder trap.
Platy fry are very small when they are first born. I'm guessing about 1/8 inch at the most. Platies can have anywhere from 10- 40 fry, maybe even more. A lot will depend on the size of the female and her age. The older they get, the more they seem to have at one time.
Again, conrats on the fry and I hope you get as much enjoyment out of them as I have.