My favorite dildo regarding how hard I cum is 8x6.5 but I cant take it all and can only use it once in a while. My daily driver is 7x5.5 and it's also great, I can spend more time with it.
I've had great sex with men of all sizes, but I prefer a bit bigger than average in general. Like if I had two identical men, one was 5, the other was 7, I'd pick the man with the 7.