I was about to say why not a 10".
They would fit your concept rather nicely. They're in between a 7 and a 12 (obviously), their short / small but easy the mix with, but they also have charm about them that gives them a kind of individual personality. I have thousands of 7s and 12" singles and lps, but I only have a handful of 10"s, some Carl Craig singles and a bunch of early dancehall and lovers rock eps and singles.
I imagine getting 7s pressed is a lot cheaper, plus you can go with a plain paper sleeve to cut costs further. But if cost is not such an issue and you want a kind of bespoke, limited, individual type thing, then I would consider 10"s.
-
Apart from that I like the concept. You're kind of trying to change how people Dj though, which will be a tough one if you're the only label doing it. As another poster pointed out upthread, Djs often make a choice, determined partly by what styles they play, to go with either 7s or 12s and then often stick to that religiously. Even scenes like juke/ghettotech where they're playing dance music and mixing rapidly between songs, they opt for 12s because they're easier to handle and it's just part of dance / hip hop culture to mix with 12s.
Good luck with it all. Always good to try something new IMHO.