I tried a search on "frame size" but there are so many threads on "what frame size should I get" it is very difficult to find the answer to my question. What dimension is "frame size"? I see bicycles for sale on craigslist but I was never really sure what dimension the frame size is when they mention say 22 inch frame.

What dimension is this measuring?

It's almost always referring to the length of the "seat tube"- the tube that runs vertically between the cranks/pedals at the bottom and the saddle.

More specifically, the length from the center of the crank to the top of the seat tube.

This doesn't equate directly to a dimension on the rider. It's more like knowing your shirt size. And note that mountain bikes, for example, are normally used in smaller frame sizes than road bikes, so you don't just have one number that tells you what bike to use in all cases. A lot of cruiser-style bikes are normally sold and used in smaller frame sizes than road bikes.

A 22" frame is a big bike.You can tell.Wal-mart bikes are between 15-17 inch frames for what they call adult bikes.

Given the length, that is what I kind of figured. Thanks.

Read what Sheldon Brown said about it...

https://www.sheldonbrown.com/frame-sizing.html#size

Technically, what you should know is your standover height (generally the height from your crotch to your feet when standing up straight), and then what that corresponds to when it comes to frame sizes.