Size is everything.
I mean, look I know that that's not really what you want to hear , but when it comes to invitation enclosure cards it's true.
Whether you're looking to design your own or are getting a custom suite designed, understanding the different enclosure cards and the reason behind those sizes could really help out as you go through the design process. So let's break it down, shall we?
This one is pretty standard at 5x7 in, and this is pretty much the golden rule. This is because it is the perfect size for making sure all your details can fit onto the card, but still not considered oversized when mailing it out usin ga A7 envelope. You can usually keep it within the weight limit for regular mail, so that you can use standard postage too (Forever Stamps are your friend).
With that said, we've done weddings with a long rectangular invite or a quadfold that was slightly smaller—our suggestion though, is that the reason for deviating from this size should really make a design statement. The different sizing could make things a little more complicated, so it should definitely be worth doing it!
The Details card is an important one because this is where you can communicate to guests things like information about your registry, transportation, directions, etc., which would otherwise not fit in the main invitation. If you just simply wanted to list a few details like your website, we would suggest the A6 size, which is 4.5x6.25 in. Since that's a standardized size (notice that it even has a name!) it's a lot easier to find pre-cut paper for those planning on printing yourself, and it's a nice size that really tucks right in when stacked with the main invitation.
But say you had a lot of information or planning to do a map, then sizing up to a 4.75x6.75 in would ensure that none of the smaller details would get lost.
If you were going the more traditional route with a return envelope enclosed for the RSVP card, the standard is 3.5x5 in since it fits the little 4-bar envelope folks usually use. But this is a rule that we love to break.
Have you thought about making your RSVP card into a postcard? This allows you more space to play with, and eliminates the need for an envelope. You can also have additional room to collect information from your guests, like if you were having a weekend wedding with lots of festivities you can get their attendance for every Sunday brunch or Friday night yoga, and even note important guest information like repeating that it's an adult-only reception (a little reminder goes a long way). It also allows you to have fun with the back of the postcard, where you can do a fun design like a quote you both love or ask for more recommendations on song choices, message for the couple, etc.
Whether you're going traditional or funky, enclosure sizes are something you may not immediately think about but has a huge impact on the scope of your project. Make sure to ask your designer about size suggestions, or research the paper you want to print on if you're doing it yourself!