Miniature paintings can be almost any "smallish" size. I've seen amazing work by Wes and Rachelle Siegrist as tiny as 1 inch by 1 inch. (they are world-renowned artists specializing in miniature paintings.)
Fortunately, the next two miniatures I am painting are substantially larger. Requirements for this upcoming exhibit and sale have the size set at 108 square inches or less (the equivalent of a 9" X 12".)
It would be easiest to do a simple animal portrait and call it good. Guess what I am going to do .... yes, let's paint a full reclining leopard in the shadow of African vegetation.
The painting is 8" X 13" (104 square inches.)
Below I have the background roughed in and some base color for the foreground grass. I like to paint the spots first to give me a feeling of the cat's muscle structure. Once I paint the other colors of the fur, I will repaint the spots and blend the spots' edges into the rest of the fur.