What size frame on average would a 5'9'' rider need?

Medium or large.

What about in inches?

18-19

that much wow, i was looking at this https://cgi.ebay.com/ws/eBayISAPI.dll...e=STRK:MEWA:IT

No way. 18 - 19" is too large for a 5'9" rider. I'm between 5'10" and 5'11" and I ride a 17.5. My friend who is around 6'1" rides a 19". I'd think that 16" may suit you well, but I would never recommend buying a bike without riding it (or one just like it) first.

I don't ride a mountain bike, but I agree. I'm 5'9" and I was told 16-17" is what I should look for if I want a mountain bike.

Got numbers crossed 17-18, but also remember that there is not a size standard. To a person who is 5'9", company X's 19" frame could fit about the same as company y's 17 inch frame. Heck I'm 6'2 and I ride a 15.5" Kona Stuff, the bike fits me perfectly.

Iam between 5'11" and 6', and I just got a 19" Gary Fisher HKEK. On the 17.5" frame I had the seat post all the way up, and felt like I was riding downhill. I have a little more than and inch of clearance when I stand over the bike. It also feels much more comfortable...

Overall height isn't the key for sizing a frame. Your inseam measurement is the critical first step.

L8R

Very true, although I would be surprised to see any 19" bike comfortable for a 5'9" rider.

Not true. I've seen women with inseams close to 34" that fit "leg wise" on a 19" frame.

Remember, these are just general guidelines to bike fit. Without all your measurements or you standing in front of me, it's hard to judge.

For example, I'm 6'0". I should ride a 19" or a 20" frame. NOPE! I've got short legs and a tall torsoe, I can ride down to a small frame(15" or 16"), but prefer a medium. (17" or 18").

For a good idea of what's involved, check out www.wrenchscience.com.

L8R

True but tall people usually have greater inseams than short people. It is a predictable standard. You don't see many 5'9" people with inseams that are 34".

Test ride the bike you want at a goog LBS. There's no better way. On a Trek 4300, I felt most comfortable on a 16", on the Hardrock I liked the M, which is 17" or 17.5. I am 5'10".

If you are 5'9 with a 34" inseam, I bet the pants don't feel so good around your armpits ;-)

Anyways typically 5'9 will be 16"-17" MAYBE 18" but I have yet to put anyone under 5'11/6' on a 19". Heck I'm 6'1 with a rediculous inseam and ride 19's in most brands.

What's more important is the actual measurement from center to top of top tube. Brand A's 17" frame can be off by as much as 2" as compared to Brand B's 17" frame in standover height because the amount of seat tube extention above the top tube varies among diff brands. Most mtb frames are measured right up to the seat collar....and this to me is absolutely useless. But this is almost always never specified on spec sheets.
What's important next is the top tube length as this should be somewhat in line with the length of your torso.
I've always purchased my road or mtb frame according to center to top of top tube measurement to make sure I have plenty of clearance/standover beneath my crotch.

well i'm 5'10 and i ride a size 17 frame, its should about right