Size, Color, Content
Many times, people worry more about the size or color or their research cards than they do about the content of the card. A 3"x5" card can work just as well as a 5"x7" card--unless your handwriting is very large. And whether or not you prefer lined or unlined cards or cards of various colors is irrelevant as long as the choice makes sense to you.
The content of the card is far more important. And the first rule is to only write one key concept per card. Otherwise, you will not be able to effectively shuffle cards around when you go to write your paper.
