Size is the single biggest lever on price, because size is really area, and area is really time. A design twice as wide and twice as tall covers four times the skin, and the artist is still paid by the hour.
Most artists price a very small piece as a flat minimum, then switch to an hourly rate once the work runs past an hour or two. That is why the jump from a coin-sized piece to a palm-sized one feels so much bigger than the measurements suggest.
Placement matters just as much as size. The same drawing on a flat outer forearm and on the ribs are two different jobs, because one surface holds still and the other curves away and moves with every breath.
The ruler above is our model for turning inches into hours. It is an estimate, not studio policy, and you can see it applied line by line in Show the math on any result.