Your save-the-date can take any size or form. If you decide on mailing cards out, then your cards can be the size of a postcard, or even smaller. Make sure the size does not cramp the wordings together.
When you decide to include photos in your cards, it is advisable to use photos that were taken after your engagement. It could be about how your life has changed, or how the both of you have walked thus far.
To save yourself time with licking the envelopes, you might consider stickers. You can purchase round stickers with your last initial or a rose or heart. These can be found at the stationery store or where you are purchasing your invitations. Another option is to purchase glue sticks to seal the envelopes.
Save-the-date stickers are also a good option for your guests to stick them into their little organizers or appointment books. Remember your bright and bold colors to catch their eye!
Another well received way is to print a mini calendar page with a 3-hole punch so your friends can attach it to their own calendars. Of course, you would have to circle or decorate the day that belongs to you.