This is brilliant, thank you so much!
For anyone home printing, it’s worth noting that (using the UK post office as the basis) standard letter size goes up to 24cm x 16.5cm which is to say that’s how big a postcard you can make without needing to worry about putting anything other than basic postage on it. Most postcards are smaller than that, but for anyone who wants more room that’s how big you can go without anything become complicated in the sending process. (You’re advised to check your own postal services if you live elsewhere of course).