Originally icons for SU where 24x24 and 16x16. However, when the toolbar system got updated for SU8 on Windows they technically changed to 32x32 and 24x24. When I say "technically" it's because the icons shipped in SU wasn't visually larger. They just got updated with extra white-space padding.
The API layer automatically applied padding to smaller icons.
Now, since then my history is a bit fuzzy. I don't recall if out icons now are actually larger. But you could, via the API make a small icon fill out a full 24x24 size. But you should check if the outer edge might overlap with some of the button borders. Verify that they look similar to the rest of the toolbar icons in SU.
Also, I observed recently on OSX that different sizes will might look fuzzy on a low-res monitor. I think the larger icons looks better in the main OSX toolbar, but will look blurred in the floating toolbar.