http://s6c.joebattlelines.com/toys/.../ironhammer.htm
Scroll down towards the bottom for comparison pics of the Iron Hammer Mech and an ARAH figure. The cockpit can be modified to fit 3 3/4" figures but it's still a pretty tight fit.
The 8" Metal Mayhem Mech for the 8" figures is waaaay too big for 3 3/4" figures.
There's also another 2.5" mech that is a smaller version of the VvV Joe Mech but it has a figure permanently attached.