It will work both with 100x100x3 and 80x80x 3 or 4mm. Depends on how strong would be your gantry, what spindle you will like to mount and how further you will like to push it.
I suggest you make your gantry from 2 pieces of the same profile you will use soldered together, hence 200x100 or 160x80 or do it from aluminum, the way Dean makes his machines, with a ball screw hidden in the gantry, depends what means you have.

The drawings look very good. Now if you aim at certain weight or to be able to pass through normal 1m wide door, don't make it higher . if you want to make it higher and at the same time light, just make the legs longer. Also if length, weight is a problem, you can lift the 4 lower beams to the middle of the M , and remove the middle leg below this point, this will not compromise the strength.

If you aim aluminum and hard stuff mainly, maybe a gantry like Jonathans will be better, look at his exemplary build