Easiest way to change the sizes is download something called , image resizer its a windows xp powertoy , just right click any image file when you apply image resizer , and go to custom and enter ur size , u can also grab lots of files at once using this , and resize a whole folder of images , make sure to tick only resize originals dont make copies bit and ur good to go. Unless u want copies of the bigger files but it means renaming every single copy.