I think 5-7. I've never tried bigger, though, and would certainly not turn it down. I sucked a guy off once that was probably 4". Smallest I've had, but it was fun being able to go balls deep without any trouble at all!
Ever since I started sucking cock in my late 30's around the year 2000, I've always preferred guys my length of 8" or larger. I've always had a thing for big cocks so the bigger the better as I also prefer a challenge. I love to deep throat and those guys with 10" cocks love watching their cock disappear into my throat as I make sure their entire cock gets my full attention.
This really does not apply to me, but with some men the girth of the member increases with the length. A long (~7+ Inches) with a girth of about 1.5 inches should be comfortable for most fellatio adventures. However members with a girth over 2.25 inches or more might be uncomfortable for the average size mouth.