How to choose size of thumbnails

Here is the thing, I was having trouble with uploading images on my posts, refer to this topic for explanation:
http://wordpress.org/support/topic/183195?replies=15

Long story short i upgraded my nice and old wordpress to 2.5.1 and i soon discovered the problem i had could be fixed without the need to ugprade. I uploaded and image and posted as thumbnail just to discover the resize for thumbnails had changed. So I pretty much wanna know if there is a way to set the resize size for thumbnails or is there a way to downgrade my wordpress or go back to the old upload format ><!

This is my website, on previous posts you can see the ‘size’ of the thumbnails vs the size of normal and thumbnail on the curret version

http://www.gamerkun.com/