The first already exsist, and can be found in the settings, the second exsist in a mod called hypertus, and the actual belly size to litter size can also be found in setting. if you download foxlib and it's extension, of witch can be easily found when sorting by name, the extension being foxedBDCC, you can also edit the settings of hypertus through the pause menu, and foxlib is just useful in general. I recommend using the built in mod launcher, this can be found in settings as well, being the first option in the menu. if you need anything else, just give a.. bark? You get the idea.
Hope this helped.