I need to replace some floor & ceiling plates that go around the pipes for my old radiators. I found one place that has several sizes but I don't know what the sizes mean. I bought a few that are 3/4". They're very snug against the pipes. Does the measurements mean the hole size or does it mean something else?? They look like a metal donut with a hole in the middle.

 

Probably the easiest way to get what you need is to determine the size of the pipe and ask for that diameter - specifying the type of pipe (iron in your case).

Most of the flanges I have in stock are carded with labeling saying (for instance) "fits 1/2" iron pipe".

The actual diameter of the hole for the above illustration is 3/4", because iron pipe is measured from the inside diameter, not the outside.