I have not played Heavy Gear, either, but I'll still put my 2 cents in. http://theminiaturespage.com/boards/msg.mv?id=369504
3rd picture from the top has Rebel Minis TomCat and Earth Force infantry (15mm).
Rebel Minis also has Vipers and the Ursa walkers.
I think they range from smallest to largest:
From a few searches for "battlemech size comparison", a mech is around 5-7 inches tall when using 15mm infantry.
Heavy Gears would be about 2-4 inches tall.
Diplomacy is the weapon of the civilized warrior.
-- A. T. Hun (Robert Aspirin)