I jumped the gun a bit guys. I did little research, bought on impulse and then read more after. All of the bicycle calculators and reading i've done points me to a 19 or 20" bicycle. My buddy has an Iron Horse sinister 6.1 which fits me well, but its been quite a while since i was on a mountain bike. I just ordered off of nashbar a "Diamondback Overdrive Expert 20" ". I got it for a great steal and have been looking forward to putting it together. But reading and such im confused. One forum told me i should ride an 18". I'll mainly be taking some calm trails nothing hardcore and street riding. I'm 6' with a 32" inseam. The bike has a 31.5" standover. With my feet 6" apart and measuring 31.5" i had about 1.5-2" of extra clearance. Someone put my mind at ease! Shipping was 45$ so i'll have heart failure if i have to send it back. It maxed out my funds for now, so parts wont be able to be bought for a little while.

At 6' you should be fine with an 18" frame. I am 6'3" and have a 34" inseam. A 19" was my frame of choice for 15 years. A tad smaller frame is quicker and I think more responsive. You can always adjust for sizing with a stem or seat post. You will have a great time. Post some pictures when you get it put together.

The OP got a 20", not a 18".

It's hard to say without knowing the geometry of the bike you bought but I'm 6" and rode a 20" hardtail from '04 to '07. I had the clearance and used to prefer longer top tubes with short stems.

So if you have the clearance and the length feels okay, you'll be fine.

[TABLE="class: specs_geometry"]
[TR]
[TH]Size[/TH]
[TH][/TH]
[TH][/TH]
[TH]20" Large[/TH]
[TH][/TH]
[/TR]
[TR]
[TH]Seat Tube Length
[/TH]
[TD][/TD]
[TD][/TD]
[TD]20.0"[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Standover Height[/TH]
[TD][/TD]
[TD][/TD]
[TD]31.5"[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Top Tube Length
[/TH]
[TD][/TD]
[TD][/TD]
[TD]24.5"[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Head Tube Length[/TH]
[TD][/TD]
[TD][/TD]
[TD]120.0mm[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Head Angle[/TH]
[TD][/TD]
[TD][/TD]
[TD]71.0°[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Seat Angle[/TH]
[TD][/TD]
[TD][/TD]
[TD]73.0°[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Wheel Base[/TH]
[TD][/TD]
[TD][/TD]
[TD]1112.0mm[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Chain Stay Length[/TH]
[TD][/TD]
[TD][/TD]
[TD]440.0mm[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Fork Offset[/TH]
[TD][/TD]
[TD][/TD]
[TD]38.0mm
[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]BB Height[/TH]
[TD][/TD]
[TD][/TD]
[TD]313.0mm
[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Stem Length[/TH]
[TD][/TD]
[TD][/TD]
[TD]90.0mm
[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Handlebar Width[/TH]
[TD][/TD]
[TD][/TD]
[TD]670.0mm
[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Crank Length[/TH]
[TD][/TD]
[TD][/TD]
[TD]175.0mm
[/TD]
[TD][/TD]
[/TR]
[TR]
[TH]Seat Post Length[/TH]
[TD][/TD]
[TD][/TD]
[TD]350.0mm
[/TD]
[TD][/TD]
[/TR]
[/TABLE]

Not that easy...just having the measurements is not enough... it all has to do with the geometry of the frame, the fork etc. Also is it a 29er or 26 wheel bike?

Too late, you bought it. Make adjustments and it will be OK. Next time, you will know exactly what is best for you after having ridden awhile...hopefully you will figure out its not just the height you need to worry about but the reach as well (i.e. top tube length).

So does that mean someone who is 5'8" can ride a 20 and a 6' can ride a 16? Is there no set rule of thumb?

Except for stem length, those numbers suggest a bike that would fit me nearly perfectly (6'1", 34" jeans inseam). The numbers also suggest a 26"-wheeled bike.

Pam, Pam, Pam..... Geometry's there, dear one. 71/73 degrees, 38mm fork offset, standard XC geo. Probably 100mm fork, if not rigid.

I can't say the OP here is experienced AT ALL with bike fit, but he's got a good start, IMO. A little help, and he could have this bike dialed in in NO TIME. I'd do it for/with him, if he was local to me.....

No. Not what it means.

Brand websites often have "sizing charts" that suggest what frame size of theirs will fit a certain height of rider. The first one that comes to mind is https://www.santacruzmtb.com It's a good resource.

So dx-man, all is not lost for me size wis? Also, it's a 29er

You'll be fine. Just ride the thing. If you don't like it in a year, sell it and get something else, perhaps something smaller. (For what it's worth, I'd put you on a 20" anyway.)

Bike is awesome. Perfect fit. Had it on a long ride tonight. Will fine tune it tomorrow

You have to post some pictures now. Serious dude!

It's crappy here right now. When I get out into the woods I'll be sure to post pics!