Recently users have been complaining again (see [1]) about too small file chooser dialogs, a bug that had just been fixed in GTK 2.12.

Looking in the code for changes, I found these commits, which apply to gtkfilechooserdialog.c. They were designed to solve bug 543308, which was in turn triggered by another commit intending to fix another bug. So maybe we should be careful here! ;-)
As these changes are the only ones that occurred since 2.12, I think they're the cause of the problem:

------------------------------------------------------------------------
r21163 | federico | 2008-08-20 03:11:20 +0200 (mer 20 aoû 2008) | 3
lines

[PATCH 2/7] bgo543308 - Remove obsolete sizing code in
GtkFileChooserDialog

------------------------------------------------------------------------
r21162 | federico | 2008-08-20 03:10:59 +0200 (mer 20 aoû 2008) | 3
lines

[PATCH 1/7] bgo543308 - Let the file chooser widget be in charge of its
default size; don't force it from the dialog


In 2.14, two issues have appeared:
- the file chooser dialog is resized *after* being shown, prompting the
window manager not to center the window on the screen (no speaking about the bad effect of the dialog moving on the screen)
- some users report problems about the dialog being much too small (see
screenshots on the Ubuntu bug report)

About the latter, I'm not able to reproduce it and it seems to be quite random, with some users getting it almost everytime, others one in a while (I saw it once, and never since then). We may still assume it is triggered by the first issue, for example if the window manager is not resizing the dialog as it wants to change size at the moment it is being displayed (this happens under Metacity and Compiz, but with changing frequencies, according to reporters).

Quite strange anyway, but the current resizing design is bad. The window should be shown after being resized. If you give me some hints I can investigate the issue.


1: https://bugs.launchpad.net/bugs/75324