Assuming a 35 GHz radar, you'd need an antenna aperture of over 10 m to resolve 2 in at 50 m.

TTFN

What resolution/What frequency range could I get with 2 meters? Is there an equation for the relationship? And is synthetic aperture a possible solution?

Down Range Resolution Basic: Delta D = c*PW/2

PW=Pulse Width

Down Range Resolution With Pulse Compression:(linear FM, etc.)

Delta D= c*PW/(2*BW); Where BW/PW = pulse compression ratio

Cross Range Resolution Basic: Delta Dy = Lamda/Dia

Lamba= wavelength, Dia = Antenna Diameter

Cross Range Resolution Side looking Sar: Dia/2 minimum.

Cross Range Resolution of Spotlight SAR: Theoretically Limited by Pulse compression resolution, motion compensation, system noise etc., but basically regarded the highest resolution mapping method. Need a minimum of 30 degrees delta between your velocity vector and look angle to be practical.

Your requirements are basically not feasible. To distiguish different material types using RF would be millimeter-wave radiometry, limited in angle resolution to the basic, no down range resolution, and high integration times.

I have had my fill with the glassy eyed DARPA types that just expect to much from the realities of physics.

BTW, Try looking into LADAR, This may be your best hope. Using RF at any frequency for this app. will be an expensive, painful, and probably failed endever.