thanks for your replies guys
i have read a bunch of threads; opinions vary from "a good 5 inch is fine for FBO" to "the 7 inch is the way to go" to "be careful with pressure drop in an IC too big for your requirements"
i think on a sweltering sydney summer day, with my flash tune pushing things a bit hard, i'll be happy to have gone 7". also, i think 7" may be easier to sell if that needs to happen one day.