Is she a maiden mare?? If so, don't put her to anything bigger than 16hh if you want to play it safe. Sometimes if you put them to a bigger stallion the foal is too big for the mare to deliver and you can run into complications.
Is it worth putting all the time and money into breeding your mare as well? It might be a better idea to just buy a youngster. Not trying to dampen your spirits here but unless she is a super good mare with lovely paces and talent, I wouldn't be breeding from her. There are far too many backyard breeders around that should stick to buying from professional breeders.
She looks like a sweet little mare, I would love to see a video of her moving though. Back is a little long and straight, with quite and upright shoulder and pasters. I'd be putting her to something a little lighter build, possibly tb with a shortish back and good slope in the shoulders to try and correct her build in the foal.
How much thought have you put into this?? I think you need to sit down and write a list of expenses. You're looking at around AUD $5000 before the foal even hits the ground, then there's the care of both mare and foal after delivery, extra feed for the mare then for the foal in a few weeks, added vet costs etc. Then there's the feed for the foal once it's weened plus agistment if that's applicable. You're looking at AT LEASE AUD $3000 a year for the first 4 years of the foal's life before you can even ride it. That's almost 20k by the time it's ready to break, then there's the breakers fee's as well!!!!!
And then you don't even know if it's going to be good enough for what you want. Personally I think it's better to buy a youngster than go through the stress and costs of breeding. Unless the mare is extremely talented with a fantastic confirmation and temperament and you use a very good stallion, the risks far outweight the benifits IMO.
Think deap and hard before you put your mare in foal, thats all I can say.