Here is a picture and a table of some sizes they come in.
You'll notice that there are 2 numbers listed for size. the first number (8-32) is the size bolt that will screw into it. The second number (1/4) is the size hole you have to drill to stick the thing into.
The t-nut goes on the bottom of your wood, with the points facing up. Your bolt goes down into it from the top. So when you tighten the bolt, it pulls the points of the t-nut up into the wood.