Will this breeding result in a dead foal/mare?

No---horses will only have the size they can handle, unlike cattle.

We used to have a string of racing pony's that we would run the fair circuit with. We bred those ponies to TB all the time with no issues.

My stallion is 15.1 hands, very average in build, i.e., not a bulldog and not a racey qh, just perfect between.

One year he bred 5 mares, all different sizes and bloodlines. ALL of the foals were huge at birth. The smallest mare had the biggest foal at 45" wither heigth at birth, and big butt and shoulders to match. The rest of the foals were all over 40" wither heigth and had the heavier build. This was the year I had to perform an emergency episiotomy on one of the mares, a maiden, to get the large foal out. Both survived, tho the mare can not be bred again due to the severity of the internal tear. 

Subsequent breedings have all produced very large foals. I will no longer breed him to a maiden mare.

I have bred smaller mares to larger stallions, and larger mares to smaller stallions. One breeding, a 14.1 stallion on a 14.3 mare produced a monstrous offspring, tho not at birth. That colt ended up 16hh! And then my DFP (DFP was 16+) bred to a 15.1 mare produced a 15hh offspring.

Unless you know for certain that a stallion is consistent in throwing large foals I would not base a breeding choice off of size difference in mare and stallion. 

I agree if this mare is maiden I would not be breeding to a large stallion. Just like dairy cattle (Holstein), my father in law had a big Dairy in the Valley and all the maiden heifers were bred to bulls that produced small calfs for their first time. 

Still had bulls that produced larger calfs and alot of calfs had to be pulled because of the bulls. I would check and see how big this studs babys are befor I would consider breeding to him.  

Thanks.