GPR can locate the rebar in concrete wall. However, to determine the diameter is difficult. It's depend on the size of the rebar; the thickness of the wall; the frequency of antenna you use; also the inversion method been applied. If it's possible, I suggest you do some physics experiment, like test different size of rebars and compare the received GPR signal. Or you can try full waveform inversion because the resolution of this method can be achieved to subwavelength.
Surface penetrating Radar doesn't have the resolution to determine bar size even at the higher frequencies (2.6GHz). You can estimate the diameter near face steel with radar by messuring the depth of cover as accurately as possible. Then at the same location measure the depth of cover to the cross bar. The difference in cover will be the approximate diamater of the bar.
Alternatively, you need to use a cover meter which uses a pulse induction method to measure the bar diameter. Typically these are not very accurate +/-20% dispite what the manufacturer may say. You need to be careful that the neighbouring bars are not too close (typ >200mm centres), that the bars are not too deep (typ