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Yep, essentially the outside diameter of the bottom of the eyepiece that inserts into the focusser draw tube - it's called the barrel.
There are some old focussers smaller than 1" and there are some new focussers designed to fit imaging cameras that are 3" (or even more!!).
But most focussers these days will have a draw tube designed for 2" barrel eyepieces. Some focussers are designed for 1.25" barrel eyepieces, but these are not very common in other than the cheap-end scopes.
So most eyepieces that you can buy come with a 1.25" barrel or a 2" barrel. Some (Televue is an example) come with a dual barrel that will fit into either size focusser. Usually, however, if you have a 1.25" barrel eyepiece, you use the 2"-->1.25" adapter that would have been supplied with the 2" focusser.
So do you want to buy 2" eyepieces or 1.25" eyepieces? Sometimes, for convenience, you might stick with one barrel size (2" so that you don't have to fiddle with the adapter), but most people don't mind doing that. Sometimes you cannot avoid going to a 1.25" barrel to get the magnification and field of view that you want for an object (eg. planets).
Intuitively, you would think that a longer focal length eyepiece (eg fl of 30 or so mm and longer) with a wide apparent field of view (say AFOV of 68 deg or greater) would only be possible in a 2" barrel, and, guess what, you would be mostly right. But narrower AFOV eyepieces (eg,. plossl at 49-50 deg) can come in a 1.25" barrel for a long focal length (eg. Televue's 40mm fl plossl has a 1.25" barrel.)
So if your focusser takes 2" eyepieces and you are happy to use the 2"-->1.25" adapter, don't worry about the barrel size.
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