Lots of factors to consider with this one I reckon, first and foremost is have/do you put your SB in the face of the bigger fish or not, are they where you are fishing? Then there's the fish du jour i.e. what the fishes may be feeding on, or take a liking to at that time..but generally matching the hatch produces more strikes so if there are lots of big fat pillies being chomped - using a bigger SB along those lines can entice the bigger fish to feed aggresively. When smaller baitfish or worms are on the menu - downsize to an anchovy style bait or worm action (dragging SBs slowly along the bottom- Rod Holder works wonders
). Also if you use scented SBs (just like real bait) smaller fish tend to get in and 'suck it and see'. If you are anchored and using berley the small fish will crowd in - so they will be the first to attack - the biggies tend to hang back further and watch for longer.
How enticing is the action you impart - the movement, the panic or injury vibe, the wriggle or inherent motion of the SB itself?
There is no difference in price between 5" or 7" Catch SPs (I assume those posting saying there is are referring to Gulp, Slam or other brands). The additional cost of manufacture for a 7 inch is very minimal to a 5 inch, yet some brands charge up to 50% more.