I couldn't find an answer on how to change the size of image thumbnails once you upload them, so i figured out how to do it on my own. All you have to know is what dimension you one your thumbnails to be and wordpress does the rest.
Really Simple fix that involves changing 2 lines in the admin-functions.php file. In my case I wanted all my thumbnails to be 250px on their largest side.
Open the wp-admin/admin-functions.php file and find:
$image_new_width = $max_side;
$image_new_width = 250;
$image_new_height = $max_side;
$image_new_height = 250;
Upload the file to the server and that's it. I hope this isn't covered somewhere else, but I could not find it anywhere. Change the 250 to whatever you want your thumbnails to be.