I've been doing some poking around with DFHack.
A dwarf with 112% height and 110% broadness gets a total of 123% increase to base size 73800. So they stack multiplicatively (1.12 * 1.10 = 1.232) rather than additively (1.12 + 1.10 = 1.22). (Although she has a smaller current size despite having above average strength and seemingly normal fat; I can't figure out why.)
Rhinoceros people do inherit all three BODY_APPEARANCE_MODIFIER tokens from the animal, so the max would be 110% * 110% * 110% = 133% (1.331) to base size 2041550 and would be described as "very large". (Or 129% if the 110 is not inclusive to the extreme range.)
A large (110% total) rhinoceros woman with 1688500 base size and the (max starting) 2000 strength has a current size of 1734310.
So a max starting base size and strength rhinoceros person might start with size about 2087360, I think. Still short of wearing elephant person armor by about 150000 and even doubling their strength to 4000 (the soft maximum) would seem to leave them too small. (Assuming size gain from muscle is linear.)
The fat template does hav THICKENS_ON_ENERGY_STORAGE similar to THICKENS_ON_STRENGTH for muscle, but I still don't know if or how that affects size, so that might be part of the current size of the rhinoceros woman above. (Though, adventurers always seem to start with 500000 stored fat, which seems to be the average.)
As for BODY_APPEARANCE_MODIFIER, from
this post I'd guess the chance of a 133% to be: 1/6 for the highest range, times another 1/6 for the highest number in that range, then cubed for getting it on all three dimensions, so maybe 0.002% chance. (Though it depends on which numbers are inclusive to which range.)
I've also noticed a bug with the way size is included in the appearance description. A dwarf with both dimensions in the range of 96-104 will not hav any mention of size. This includes not mentioning if they are muscular or fat. But then if either dimension are out of that range but still in the range of 91-109, they will then be described as of average size (or just muscular and/or fat if either of those apply). Only when either or both dimensions are outside that range are they described as short, tall, thin, broad, small, large, stout or lanky (and muscular or fat as applies).