Best thing is to talk to the maker of the cart or the dealer, your horse is 30" tall so that would be an A size.
Thing is that it depends on the cart what shaft length you need. Reason is that some carts have a low single tree for instance and then you would need to have the horse further forward so it does not kick the cart with the back of its leg. In other words your horse needs room to move. We have a pacific smart cart and it always amazes me how close you can get the horse in because the single tree is relatively high and the front of the cart slopes back. Some carts have a more upright or lower front and the horse needs to be further forward, that would mean longer shafts.
As you see there is no simple answer to the question.
Beter carts like the pacific smart have the option to slide the shafts forward or back to adjust for length. then there is the width between between the shafts, also adjustable there and then there is the height of the shafts this can also be adjusted.
I know, it is mind boggling, then there is balance to deal with, this becomes even more important with a smaller horse like yours....
I think easy entry carts are not the best for trails, they are often to narrow and high thus easy to tip on uneven ground. The option of adding a second person is nice but for trail use not the best idea.
I think the HyperBike is the best cart for trails certainly considering the size of your horse.
Then the fact that you as well as your horse are green, so you may want to leave the trails for step two and start with a training cart first.
That will be more focussed on learning to drive and be safe in an arena first and then later thinking about upgrading to trails and maybe even competition....
We started with entry level and cheap used stuff, and quickly found that only the better stuff actually is worth the investment.
I agree it is expensive but then your horse cost only $25.00 a month to feed
