i would want it as a general all round to participate in most sides of riding, she is only 13.3hh so would obviously want somthing of a bigger height, but want her to be comfortable as we want her to be done naturally.
If she's a maiden I wouldn't go for more than 6 inches or so, an experienced mare could possibly cope with a little bigger. But I'm not that experienced so others will know better! Some say the mare limits the size of the foal, which will be true to some extent but the mare could have the foal v early or really struggle with the birth.....
Depends what type you want to produce. I have had dealings with two types of Highland crosses that I would recommend. I really like Highland ponies, one of my first ponies was a Highland and we had tremendous fun together.
The arab/highland made a finer (and very pretty dun) riding type pony about 14.2 that was a super all round teenagers large pony, sensible but with a bit of fire to it at the same time.
The second, which I adored, were highland/cleveland crosses. They were bred specifically as trekking horses and they were perfect, a little more height, but decent weight carriers and as sensible and honest as the day was long. Again they were all, without fail, duns and very handsome horses. We had about 20 of them ranging from 14h - 16h, if I was ever going into that kind of venture and breeding the perfect trekking horse that is what I'd breed. Oh, they weren't brain dead slugs either, although they mainly plodded about with passengers in the usual nose to tail fashion during the summer, they could, and did jump, do dressage, hunt etc.
The Cleveland stallion would perhaps be too big for your mare though
Traditionally you should breed blood to bone, rather than the other way around, so by starting with a Highland mare you have a choice of TB, arab, small warmbloods, connemara, even an andalucian type could be interesting.
They can make successful sport horses.
The only type of the above I would be wary of is the warmblood, lovely horses but often very cross bred so there is a slight danger of throwing back to the heavier side - you really don't want to go to something with too much bone or you will breed a tank.
Research the parentage of your chosen stallion to be sure that the one you have chosen is typical of the breeding, and not just a freakishly small example.