I’m not dictating the size of your postcards. I suggest copying the size of a regular postcard to make a template that you can use when creating yours.
There is a standard size for postcards in some countries – but there are no rules about size in this swap. You are free to create whatever you want!
I collect mine in a special postcard binder, so I appreciate postcards that are not huge. I prefer it when your postcards are regular size, maybe about 10×15 cm (3.9×5.9″) or a tiny bit bigger