Outer Envelope Size...so many options!!

I am also wondering about this. The envelope that comes with a 5x7 will definitely fit the card. These are typically size A7. The trick is finding an envelope that will fit that envelope! Pretty sure size A7.5 will work but finding that size is the problem. Would love to hear of others who have has success with this!

This isn't something I considered. Do you have to have an outer and inner envelope?

You don't have to have 2 envelopes - definitely not required. But typically the outer envelope has the address, stamp, and name of the person you know at the household. The inner envelope just has the names of the people you're inviting. So for example: I want to invite my cousin Bob and his girlfriend Shelly.

Outer envelope: Mr. Bob Smith with address

Inner envelope: Bob and Shelly

I am not doing an inner envelope. Saves on money and can help with postage. My invites are requiring a basic stamp since weight is low. I'm just putting guests names on front. I.e. Friend with sig other. Mr joe smith and miss jane doe

I would skip the inner envelope. You are adding weight and by getting a bigger outer envelope you are running the risk of more expensive postage.