The vintage rolodex that I use holds cards that are 2 1/4" by 4". I think there are larger size rolodex. The 2 1/4" by 4" is considered the standard size.
I collage right on the original cards.
If I have a piece of paper that is heavy, I will draw around a rolodex card as a template and then cut out the paper to size.
One thing I had not even thought about was orientation. Most of my card I use as a horizontal orientation and my rolodex is like a book. It wasn't until Brian sent me some finished cards that I realized that a vertical orientation may be more in keeping with the original flip type rolodex.(The above card is one of Brian's) I also noticed that some holes are shaped a bit different than mine. I don't think any of this matters and I think a rolodex with cards in different orientations make it more interesting.
The most frequent question I receive is how to cut the tabs of the card. I don't use a punch even though there are rolodex punches. I find that my cards are too thick with paper and paint for a punch to work. I cut out my tabs with a small scissors. I have also found that you don't need to be neat and perfect when cutting these because once they are in the rolodex you don't notice the cut tabs. Just don't cut them too big or too small.
Because I like to make these cards up quickly, I use a tape runner to adhere paper to my cards. I will also use a glue stick and I don't have a problem with paper lifting up but I know some people who this is an issue.
I will also use white glue or mod podge. When using a liquid glue, the cards can have a tendency to curl or roll. One way to help with this, is to lightly coat one side of the blank card with glue before using the card for collage, let dry and then coat the other side with glue and let dry. Then you can use this card to make your collage.
When I collage and work on a card, I work on one side at a time. Then I trim the edges and cut out the tab. Once that is finished I will work on the other side and trim and cut out the tab once again. I find that working one side and cutting it keeps it easy to keep these cards and tabs in the correct size and position.
This is an easy project but don't hesitate to ask questions.