emcee2 - I just activated the Twenty Ten theme on my test site and thumbnails worked okay and were at their proper size. Not that that helps you in any way, but wanted to let you know; maybe there's something else you have installed that is causing the funkiness. You do have a featured image set for each post, right?
picandocodigo - I am having an issue where using the array values to change the thumbnail size is not working. My code looks like:
[catlist id=3 thumbnail=yes thumbnail_size=array(50,50)]
I tried also:
[catlist id=3 thumbnail=yes thumbnail_size=(50,50)] and
[catlist id=3 thumbnail=yes thumbnail_size=50,50]
But nothing seems to work to change the thumbnail size using the array values. Using the text values of thumbnail, medium, large works fine. Is my syntax wrong or is there an issue with the plugin?