Very cute :) Congrats!
To answer your question, horses can carry up to 20% of their body weight.
Actually 20% is just optimal. Many horses (especially the hardier breeds such as cobs, heavy boned ponies, mustangs, etc) can easily hold more than that without any stress to their bodies, so long as you condition them well.
I'd personally try not to go over 25% but 30% would probably even be alright given that the horse is sound, in very good condition, and not ridden excessively.
OP, your mare is beautiful! Looks like such a sweetheart too.
Wow! What a nice mare. I'd just enjoy riding her for awhile,teach her some moves to enhance the riding experience & start rating the speed of all her gaits,so she is responsive to her rider. Then-if that isn't enough for you, think about branching out.