I have these on my bike now:
they are the 40 MM cans. Plenty big for stuff like tools and jumper cables, etc ... not quite big enough for overnight and such ... but that's what bungies are for.
They are a bit heavy ... nothing like the big cans that weigh 24lb a piece ... but these definately weight ~12 a piece. They're 18x6x10 and they look pretty streamline on the bike ...
Let me see If I can post a pic ..