I would like to start doing bits with both of my kids (3 yo and SN 1yo). I have a color printer and a laminator, so I plan to make my own flashcards.
What is a good size?
Should the description be printed next to the picture or should I only have the picture?
Any other info I should know?
The bigger the better, so use the whole piece of paper.
Put the picture on the front and the bits of information on the back in list form. You show the picture and read one thing from the list, next time show the same picture but read different info from the list.
Start with something your kids find interesting. When you are bored with it and ready to quit do something you are interested in for a while 
Read up on a Doman encyclopedic knowledge book, if you need more details. They are often available cheap through betterworldbooks.com or at your local library for free.
Once again there are no rules your kids will let you know if you get it wrong