sizes that are pretty standard are
A2=8.5x11 cut in two and each piece folded to make 2 A2 cards,,,
6x6 or 5.5x5.5
I make all those sizes,,,,,,,
the square envelopes are more expensive, for some reason, so I make a lot of A2 cards,,,,,Walmart has boxes of "invitation size" envies that these fit in nicely, with a bit of room for embellishments, etc,,,,,,,
and I also buy clear plastic envelopes that are better for larger embellies on EBAY.
the one tip I could give anyone who's used to scrapping and wants to get into cardmaking is this,,,
it's only a small LO without a picture,,,,,use something else as a focal point,,,,a stamp, an image, a grouping of flowers. whatever,,,just treat it as a tiny LO and you'll make gorgeous cards