One thing that you can do that will surprise many people is go sit in the cart not hooked to a horse have another person pick up the shafts and move them back and forth. It will surprise most people how light the cart is. remember your are on wheels.
I used to do this when i first got into minis and people were shocked when i said they pulled me in a cart.
They looked at me and felt bad for the mini.
Then i went and sat in the cart and asked them to pick up the shaft, they would take a mighty grip and a big heft, then were amazed when they could pull it back and forth with just one hand quite easily.
Pokey was 31 1/2 and pulled me very easily.
Now would I go up a mountain of course not but some of the best driving horses i have ever seen were 30 inches and boy did they haul A^*!!
My Rocky is 35 inches built like a full size Morgan tank and while he drives,he is about as lazy as it gets and is just not that into it.
I don't think it depends on size, I think it depends more on heart and enjoyment from what iI have seen over the years that is what is most important.
I remember a little guy named Bill Bailey who was 28 iches if we put him on stilts, but man put him in a harness and you had all the action you wanted! He seemed to grow to 16 hands and just strutted his stuff.
JMO
Bonnie