How can i know a galium arsniad THICKNESS ?

Is there a rule of thumb for wavelength?

200 Ghz would be about 1.5 mm.

You got no so much choice.GaAs substrates are generally 150um-300um.( there may be other particular thicknesses but industry standards are around this value)

At that frequency I'd expect the active device to be
in some other material, an epi layer on top of a GaAs
handle. Mechanical handling issues will limit how thin
the wafer can be during fabrication, any thinner you
would do with a backgrind at end of line. The foundry
would assert a minimum thickness their equipment can
run without undue breakage. Likely set up for standard
available wafer thicknesses I expect. I'm no III-V
guy but I'd expect that high frequency wants all the
semi-insulating thickness you get so as to drive down
packaging related parasitics. I don't know nuthin' from
wavelength effects or whether this is even a concern.