IMO you are thinking exactly backward. You want the LARGEST dish possible. Never go with the smallest acceptable dish for anything. I understand the need for aesthetics and all, but too small of a dish will hound and haunt you until the day you replace it.
Location does play a part, of course, but you can bet the guys that are getting it with a 7.5 are guys with experience under their belts.
I have been tuning a bit on my Birdview 8.5 lately and I have picked up a few points on the ABC-S2 channels, but I haven't tried the CBS, because the Vantage doesn't do 4:2:2.
Save yourself some headaches and get a 10' dish. And save yourself some money and find a USED one, put a little 'sweat equity' in it, and have yourself a nice dish. I intend to do the same, which is why my BV is up for sale. It works, but I want a BIGGER dish for more headroom on those weak signals.