I'm not really sure this is the place to put it.
If we make it client-side, then no matter what the GM puts into the buttons the client will override and make it look terrible. (That's a user's job after all.
It shouldn't be forced upon players though as those with low resolution displays may prefer as small a size as possible even if the button don't necessarily look good.
My preference would be for the button to have a "preferred width" that could be overridden by using a <div>
tag with appropriate style
attribute, or via a user-defined stylesheet.
Since user-defined stylesheets are needed for a variety of other things, that's probably the way to go with it...