Thanks for taking the time to report this!

Since the most recent software change

I guess that translates to 1.25wmf20 (rMWbe38f2fdc09f)

<inputbox> ignores the set width= values. For an example, see https://commons.wikimedia.org/wiki/Commons:License_review/requests/archive.

That page's wikitext says:

<inputbox>
bgcolor=
type=fulltext
prefix=Commons:License review/requests/archive/
break=no
width=22
searchbuttonlabel=&#x1F50D;
placeholder=search archives
</inputbox>

and that becomes in HTML <input class="searchboxInput mw-ui-input mw-ui-input-inline" name="search" value="" placeholder="search archives" size="22" dir="ltr" type="text"> here in Firefox 36. So the width/size is kept, at least here.

If that's not the actual problem that you refer to, more information / a clearer description is welcome.

T63526 caused this probably.

This may be caused by mw-ui-input-inline.

Compare to T92498.