My thoughts on a maiden mare that size are that a pre-breeding vet exam is necessary. Internal US and also palpation (to verify size of birthing canal, etc.). Ideally, this is done by a vet that is experienced with miniatures so they understand how much room is really required. My vet uses her hand out flat and then turns that flat hand all the way around in the area of the horn to see if she feels any "catches" or if she has to "squish" her hand a bit either when top to bottom or side to side. Don't know if I'm explaining that very well or not, but that's worked well for me.
The second thing (and it's just as important as the exam) is the stallion you choose. He MUST be incredibly refined and have babies on the ground so that you can look at those babies, their dams and then decide does he actually produce refined babies or is he just refined himself? Also look at the breeding behind your mare (as you already seem to be) and see what the general look and feel of the horses is...fine boned, stockier quarter horse types?
Then as someone else said, be prepared for the worst. You must decide prior to the breeding if you're willing to accept the risks associated with the foaling, but that is true for any size horse, not just a 27" one.
I have a 28" mare currently in foal with her first baby. I did the pre-breeding exams (TWICE - I'm a little obsessive about it
), I looked at her background and then I chose a stallion I thought would do her justice and produce the best baby possible. And having foaled out many mares, including for other people, I still decided this was more than I could potentially handle in my current set-up, so she's been sent away to a farm that has the facilities, vets, knowledge, etc to handle any situation better than I. Does all my effort ensure a smooth foaling? Nope. But I want to give her the best chance possible.
And lastly, to be brutally business-like about it and take out the emotion...I have her insured...because things can go wrong no matter what size the mare.
Now, having said all that, unless your mare is a superb example, is a national champion or has some unique attribute about her, why breed her? Why not enjoy her as she is, show her, love on her, whatever makes you happy. And if you just want to have a foal, maybe buy a baby that is just weaned or at least find an experienced broodmare that is bigger. That won't guarantee you have no problems, but it may reduce your odds of a heartbreak.
Good luck in your decision!