I don't think I would implement this as a BAT file solution for any of my clients but here is a batch file that works.
REM Zero out any previous values
set /a fileSizeHD=0
set /a fileSizeDT=0
set /a fileSizeTD=0
REM Sum up file sizes for selected files
for /f %%i in ('dir /b /a-d "\\LOC\INBOUND\LS_PRSAHD_*.txt"') do set /a fileSizeHD=%%~zi + fileSizeHD
for /f %%j in ('dir /b /a-d "\\LOC\INBOUND\LS_PRSADT_*.txt"') do set /a filesizeDT=%%~zj + fileSizeDT
for /f %%k in ('dir /b /a-d "\\LOC\INBOUND\LS_PRSATD_*.txt"') do set /a filesizeTD=%%~zk + filesizeTD
REM Compute Total KB Value
set /a Total=%fileSizeHD%+%fileSizeDT%+%fileSizeTD%
set /a KB=(%Total%/1024)+1
echo %KB%
set size="Websales File Size: %KB% KB"
echo File Size : %size% >>%logfile%