Does the size or mass of a body affect the time it takes for rigor mortis to sets in? For example: compare a 300 pound man to 100 pound girl to 5 pound animal.
One of the main factors that impacts rigor mortis is temperature. Basically, a smaller body would slow down the metabolic processes behind rigor more substantially. Also, one of the first things that happens in rigor is the stiffening of muscles. An increase in muscle mass would increase the time for rigor as well.
Refer to these links: wikipedia and another that talks about rigor mortis within the scope of forensic pathology.
Hope that helps!