well... since you ask...
er... ooops - drat! I hate that question!
but... since you asked. 5 inches or less with a circumference of 3 inches or less is smallish. Its the kind of tool that ... well... condoms tend to slide right off.
slightly longer and slightly thicker and now you are talking about average. 5 1/2 inches, 3 1/2 inch or better circumference.
large refers more to thickness than length. You can have a 6 inch appendage, but with a 4 1/2 - 5 1/2 inch circumference, you've got something in the larger category.
length is not so important after a point... you ever notice on those porn flicks how the really long ones get ripped off when they are plunging in and out - about 3 inches never really get in.. and everything just sort of bends? that's cuz the average gal's... well.. heck... the length is four inches. Sure its flexible, but only to a point. Its made to stretch widthwise - not lengthwise.
I know guys never believe me when I say they are just fine... I hear so many whistfully say they wish they were 8, 9 or more inches.. and I gotta tell you. That can really hurt and even, in some cases, start a gal bleeding. Oh yeah - that's a lot of fun! I'll pass.