back in the amateur times it was possible to define exact Crop size before and during cropping. and now it's only the aspect ratio. =(
[and on that topic – moving the crop area with the arrow keys, please]
What features would you like to see in Pixelmator Pro?
As far as I can see, it's always been the aspect ratio only in the original Pixelmator, but one workaround is to create a shape with the size you need to crop to, load a selection of that shape, and then choose the Crop tool. We do have plans to add cropping to custom sizes in the future, though!