Heaves, definitely I would not breed her. First off, her condition, or the tendency toward it, may be hereditary, and secondly I don't think it's fair to breed a mare with heaves. Being heavy in foal may very well make her condition much worse--I don't know how bad the heaves are at this point, but please know that if she has difficulty now she may be have major difficulties in the last weeks of pregnancy. (You say she is "not that bad" now but from years experience I know that is a very relative term. A horse can be not too bad today and then tomorrow be gasping for air. Some drugs which can be used to help a horse through a severe attack cannot be given to a pregnant mare.
The C-section would concern me some. I know there are mares that have had a C section & then went on to deliver foals naturally, but there are also those that shouldn't be expected to carry & deliver a foal afterward. From the sounds of it you don't know enough about your mare's history to make an informed decision to breed her again--and in that situation I would not breed her again.
As far as your question regarding breeding her to a larger stallion...that is generally NOT the thing you want to do! For small mares you should look for a small stallion--and not just small in height but stature--it may not be a problem to breed her to a stallion that is a bit taller than she is as long as he is fine boned. You don't want to breed to one that is smaller but very heavy/big boned. I would suggest that you look back through the forum pages & read some of the posts you'll find on breeding small mares...and on breeding in general. Be sure to read the posts on lost foals--foals that died because they were too large and/or positioned wrong & couldn't possibly be delivered alive. Sometimes the mare dies too.