I have not played with that cartridge much, but when I did I found it cut much smaller than I would have liked. Considering you are just hoping to put some kisses in there, 12x12 should work well for you.
My suggestion is to play with some scrap patterned paper you don't really like anymore (LOL...is that an oxymoron??!!). That way you can see exactly how it will cut at a given dimension and go from there.
Incidentally, this cart is the reason I got the 12x24 mats. I just did not like how small the boxes were on the 12x12 mat. But it has been a long time since I played with it.
Good luck!