Hit Points: I think I'll set this to a flat 1/level, no Con bonus, +5 for large, +10 for huge, +15 for gargantuan and larger. (The idea of a colossal minion messes with my head.) That would put yesterday's ogre at 13 hp.
Weapons: Instead of mucking around with weapon sizes, I'm thinking to make things easy minion damage will just max out at 1d10+Str modifier.
I'm also toying with the idea of building "monster role" classes (i.e., soldier, artillery, skirmisher), which would then turn "minion" into a template that would be added. This will take a little more thought. Details as events warrant.