using a theme, how do I resize thumbnails?

[lurkmode]

i've seen several galleries (for digiscrapping) with thumbnails that are about 200px by 200px.
even the categories and random files are sized that way.
(i'm quite sure it's CPG 'coz it says so below the galleries)
how can i overwrite the default sizes of thumbnails (also in the categories, and random files) using a theme?
(i don't like the idea of modifying the thumbnails.php, etc)

[Hein Traag]

Thumbnailsize can be set on the config page  iirc

[lurkmode]

yup but by then, the thumbnails have been created already so if i set it there, the images will be stretched.
is this a limitation of CPG?
i want the look of photopost (but of course not willing to pay so much hehe)
so i'm hoping i can display my CPG to have those big thumbnails.

[Nibbler]

Use admin tools to rebuild them after changing the setting.