You may just eliminate the three last digits of the size to convert bytes to KB, that is equivalent to divide it by 1000. If you are interested in the size in MB, the difference does not matter:
@echo off
for %%a in (*.bin) do set fileSize=%%~Za
set fileSize=%fileSize:~0,-3%
:BEGIN
if %fileSize% LSS 1500000 goto 15MB
if %fileSize% LSS 5000000 goto 50MB
if %fileSize% LSS 10000000 goto 100MB
if %fileSize% LSS 15000000 goto 150MB
goto Error
You may also achieve the comparisons this way:
:BEGIN
for %%a in (15 50 100 150) do if %fileSize% LSS %%a00000 goto %%aMB
goto Error
Note: Previous code duplicate the behavior of your own, but I think you should review the numbers you use in the bytes -> MB conversion...