As @Richard-MacCutchan has pointed out, you cannot use += in batch files you need to use the long hand version (caveat - untested)
set Size_slu = 0
set TotSize = 0
for %I in (c:\Systems\SLU\*payments*.xml) do ( set Size_slu=%~zI
set TotSize=TotSize+Size_slu )
echo total file size :%Tot_slu%
Note the brackets as I've split this over two lines, although the following should also work
for %I in (c:\Systems\SLU\*payments*.xml) do set Size_slu=Size_slu+%~zI