I’ll keep this short and sweet. The default thumbnail size is 128×96 pixels. After google-ing for hours, I decided to try modify it myself. It’s pretty straightforward to get the work done actually.

Step 1: Search for the file wp-admin/includes/image.php

Step 2: If you want to have new thumbnail size of 400 width and 300 in height, replace the value 128 with 400 and 96 with 300 throughout the entire file!

Step3: Thats it!! Enjoy!

Note: You need to make sure width/height is equals to 4/3 or 1.33 (Example: 128/96 = 1.33, 400/300 = 1.33)

UPDATE: 30 MArch 08: You do need such trick for Wordpress 2.5 as it provides such thumbnail resizing function in its admin panel.