there really isn't any practical limit either way
run as many gph
as you need to achieve a good turnover of tank water through the filter, so that is recommended 10xfor saltwater = 900 gph
so something like a magdrive 12 would work nicely for you
obvisouly you want to make sure your filter is dry "enough" ... if water is flowing so fast through your biotower that very little air is getting in there then you are loosing filtration efficiency ... and need to make the area larger.
conversely, if the area were so large that pieces of biomedia were never getting wet, well, it would be a wasted capacity, but room for future expansion perhaps.