The standart image size in SPM is 512*512 pixels. Undoubtedly some images are bigger while others are smaller. But if you work with a standart screen resolution, it is convinient to work with images of 512*512 pixels. The menu item View → Normalize is designed to replot an active image in the standart size.

For rectangular images with unequal width and height their proportions will remain and the bigger dimension will be 512 pixels.