Most cards are half a sheet of 8 1/2 by 11 paper folded in half. This is invitation size or A2.
You can always make your own envelopes out of different kinds or paper using an envelope template or a Score Pal, Envelope Punch Board or the like.
Envelopes can be bought at the craft stores or office supply stores in bulk. Craft stores do have plain and patterned envelopes to choose from.
Clear envelopes are a bit more expensive and can be bought online. Some people fold a card inside out to protect the layering. Others insert the card so it can be seen and then insert a piece of paper with the name and address on it. The address shows on the flap side with a stamp adhered over the flap line.
For bulky heavily embellished cards, you can send them in bubble wrap envelopes or you can create a box envelope. Nicole has instructions here for an A2 standard size card but you can watch her video below (she also makes one with a window). You just need the measurements to create one if you don't have a Score Pal or Punch Board. Use a dead ball point pen or junk mail credit card or a plastic bread clip/tie to score your lines.
Envelope Templates at Mirkwoods - many varieties print them out on card stock and then cut them out, use as templates on patterned paper.
You can also use the Envelope Generator and type in your measurements to get the paper size needed to create a custom envelope using the Punch Board. Maybe you don't have a Punch Board, but the paper size alone is enough to get you started.
Here is another "cute" box envelope tutorial which is a non-standard size but they offer a PDF of the template so you can print it out and cut it out. (image below)
Else's Bella Artes offers five FREE PDF file downloads for mini-envelopes. Her tip: print them out, cut them out, laminate them for re-use.
!6 Creative Ways to Address An Envelope - Mail Art Ideas (image below)
Envelope Quick Pictorial Guide follows below.