Mine are 5/8 and 1/2 silicon. I'd think 3/4 would be fine, but with harder tubeing you might not get as good a seal.
Heres a pic
Red is 1/2, Black is 5/8.
Frankly I wouldn't use anything else. With gears I filled ~ 1month (I had a tiny bleed line). Its been over 2 months and I've haven't lost any significant amount of water accounting for bleedoff. I may make it 6 months before my res gets low. Aside for the new res and clamps, there was no other change.