Cropping Medium image size

Hi everyone,

I was wondering if there is a way to make WordPress crop Medium (or Large) images like it does with thumbnails. The problem is that i want my medium images to be exactly 170×250 px but those two values are maximum sizes, so i get dimensions like 166×250 or 170×216 etc. I’d use thumbnails, but they are already being used for even smaller image versions. Any way i can get this done without a third party PHP script?

Thanks!

Slobodan