I think your prices are a tad on the low side. I look at it more in terms of how much time things take. It will take nearly as much time to do a 5 by 7 portrait as a 10 by 13 . I mean, it takes time to sketch out the subject, and that doesn't matter whether it's small or large, at least for me I put the same amount of attention to detail either way.
As for the painting/drawing time, it may be less for smaller vs larger, but not that much. For me, the addition of another subject makes it take a lot longer, so if someone wants two horses in one painting I charge somewhere less than one portrait x 2, but more than just 10 dollars more.
AND, if a human is in, well that takes a LOT! more time. So, I must ask more for a portrait of horse and owner.
So, in general, I think you should consider your time when pricing things.
I tend to price somewhat individually. I give the client a range, then after I have seen the photos I can zero in better to an exact price.
I make very little on a per hour basis. Not even a 4th of what your average riding instructor makes. But, it's what the market will bear.