One of the nicest things about making your own cards is that you can make them any size or shape you want. But if you don't want to make your own envelopes, it's important to make your cards the proper dimensions to fit a pre-made envelope. These are the three most common envelope sizes and the size of the cards that fit:
Envelope Dimensions Card Dimensions
A-2 4 3/8" x 5 3/4" 4 1/4" x 5 1/2"
A-6 4 3/4" x 6 1/2" 4 1/2" x 6 1/4"
A-7 5 1/4" x 7 1/4" 5" x 6 7/8 "
If you want to make your own envelopes, it's easy enough - it just takes time. Take an envelope the size you want and carefully open it up to form a template. Trace it onto the paper you want to use, score, fold and glue. DONE!
You can also buy templates for various size envelopes. Do a web search for envelope templates and you'll find anything you need.
Now you'll have to add adhesive so the envelopes can be sealed. You have a couple of choices here. You can use a strip adhesive where you pull off a strip of paper to expose the glue, or you can use a remoistenable envelope glue that you spread on, let dry, and then lick it as a pre-made envelope.
Don't be limited by using plain envelopes, either. If you use a scrapbook paper just be sure to leave a space for writing an address, or use a mailing label.