What "size" is my full-suspension bike?
 

I've read there is a rule of thumb for finding if a bike fits -

1. sit on bike and inseam should be 2" off ground (or, you are just able to stand on your tippy toes!)

I bought a full suspension bike at a garage sale years ago. I've ridden it twice. I had the saddle fairly high (I am 6' 1" tall). But, as far as I can tell, the bike is too big for me - the seat in its lowest position is 36" (I have a 30" inseam). So, I need help so I can figure out how to sell this to someone it will fit.

This is the full bike:
https://cimg6.ibsrv.net/gimg/bikefor...c41acd1966.jpg

Here is the seat in it's lowest position (highest = 43"):
https://cimg0.ibsrv.net/gimg/bikefor...1bd79b7ead.jpg
And the imprints on the bottom bracket, in case they are useful:

https://cimg7.ibsrv.net/gimg/bikefor...9ad0a15b69.jpg

Few bikes I ever saw listed on craigslist and elsewhere listed a size. Maybe just measure the seat tube, the max saddle height, effective top tube length and stack. Pic's of each with tape measure might help.

Few buyers will probably know what to do with that though. So don't sweat it.

"size" is seat tube length up from center axis of the BB..

Take a horizontal line from the top of the head tube/headset to where it intersects with the seatpost. Then measure from this point down to the centre of the bottom bracket. This will be the nominal size of this frame in inches/centimetres.

For the record, I don't believe that this bike is too big for you, based on how much seatpost you have exposed, but if you want to sell it then sell it.

6ft 1inch.... 30 inch inseam....Huh?

You should not be able to reach the ground while seated. I an 5'8" and my bike saddles are around 38" high if measured from the ground. You should be able to get full leg extension while pedaling seated. You slide off the saddle to reach the ground when stopping. Looking at the headtube and guessing that that bike should be pretty close for you.

utter nonsense. that has nothing to do with a bike's size or how it fits you.
if you're going to talk about inseam, you need cycling inseam or actual inseam, not your pants size. look that up. how high is the bottom of your pelvis from the ground when standing in your bare feet. that's a start. I am 5'9" with average proportions and my actual inseam is about 34". my legs might be just a tad on the long size for my heigh. if you're 6' tall with a 30" actual inseam, you might have an undiagnosed medical disorder.

most bicycles have a sticker on the seat tube of the frame that states the frame size. at 6' tall, you should be on a 19-21" frame.

unrelated—that chain needs more than oil. it needs to be replaced. it's way beyond maintenance now.