I calculated the expected beam radius at X end (and I did the same for the return beam as well), because the radius for bKAGRA phase1 could be different from that of the final configuration (35.3 mm) due to the lack of ITMs.
I used usual q-parameter and ABCD matrix formulae.

Results:
beam radius (w) at X end:  66.0 mm
beam radius (w) at PR3 (return):  45.8 mm
beam radius (w) at PRM (return):  4.8 mm

This result says that the size of the beam at X end will be almost the same as or even smaller than that of GV at the end of the arm.
I attached the matlab file used to get these results (change the extension from .txt to .m; an m file cannot be uploaded here).