I am not sure if it applies to all the themes or not, but I do see the problem on TotalBlue and IceCube (I don’t have time to try them all out.)
On both these themes, part of the problem actually stems from the fact that all the board measurements seem to be specified in ems, which is a unit based on the text size being used by the browser. Ems are really great for making a layout that adapts to a user’s browser settings, but they suck when applied to the fixed pixel dimensions of images. For instance, I must use a smaller default text size than Virtual Stranger, because Firebug tells me I only have room for a 153 pixel wide avatar.
So as you can see, simply changing the width of accepted avatars won’t necessarily fix the problem. Not without making the accepted avatars much, much smaller anyways. Now, if one does not want to eliminate the em-based measurements, there are two fixes that spring to mind. They are both armchair webmaster type things, though, so feel free to ignore them.
First, the stylesheets could set a max-width on avatar images. This means the avatar could be constrained to our relative em-based measurements. That will keep the avatar area from possibly getting too big (aesthetically, anyway). But it also sorta violates the general principle of not re-sizing images. But in this case, I think violating that principle might be okay.
The other option is that the avatar area could be set to have a min-width and any margins around the avatar itself should be transferred to the padding of the overall area. Of course, this would require a bit more math, especially if one is wary about the forum breaking in IE version-why-the-fuck-are-you-still-using-that?-upgrade-that-shit-already. This stays clear on the image resizing principle. On the other hand, folks who, for whatever reason, use a really tiny text size might see the avatar area take up more space on their screens than would otherwise be pleasant.
On a side note, Firefox does not display scrollbars on these elements. It just clips them. Which I kinda find annoying in other “overflow: auto” contexts, personally. Some sites kinda depend on the scrollbars being there to make all the overflow content visible. But in this case, the scrollbar-less clipping is actually preferable.