We have been selling small coloured cards for notes for many years. They are of a size chosen to fit into handy boxes or wallets. The card we use is 160gsm, which is very much on the thin side of cards (about half as thick as a good business card). Some people call it thick paper. The advantage lies in the lack of bulk - useful if you are using lots of the cards.
The idea of revision flash cards is to write a question on one side, the answer on the other. |Preparing the cards is a useful start of the learning process. As you self test using the cards, some time later, you place the cards that you correctly answered into one pile, and those that were causing difficulty into another pile marked "more revision required". Alternatively the poorly answered cards are returned to the pile, to come around later. Educationalists who have studied this method claim that it is very effective, especially if repeated over a period of several weeks. That may be boring, but if it works it is also rewarding!