The conditions described – Sativa-heavy strain, growing outdoors, south facing and plenty of sun – make a tall plant very likely.
There’s not really an average height for a given strain. The heights listed on the Sensi shop site can be seen as the largest practical height for each strain, but all can be grown bigger or smaller than this. It’s also why the outdoor strains are listed with heights up to 3m. It would be possible to grow most indoor strains this tall, but few indoor growers would find that useful.
Indoors, growers can control the size of plants in several ways, chief among them being light cycle manipulation. By controlling when plants begin flowering through changing their light cycle, indoor growers can tailor any strain to their needs. For instance, taking the extra height gain of flowering Sativas into account, very tall Haze strains can be flowered as small clones, leading to finished plants under 1m tall.
Outdoors, with the naturally changing light cycle, this kind of control is much harder. Plants grow in the long days of spring, then flower when the days get shorter. Outdoor plants usually start seriously blooming between the end of July and the middle of September.
Controlling the height of outdoor plant can be done by selecting strains which gain less height, by starting seeds later in the spring/early summer, so they have less veg time before flowering kicks in, by restricting their root growth with smaller pots (though this won’t always stop the plant getting tall and skinny, and will usually reduce yield), by pruning, by bending the main stem of plants and by using autoflowering varieties (whose final size is more or less predetermined).
To cover the requirements in the OP, the best bet might be to get a strain like Jamaican Pearl, Durban or Early Pearl (possibly even Early Skunk, though this has less Sativa component) plant them in March or April as normal and let them grow naturally.
When plants get to around 30cm, their plants can start to be gently bent with strings attached to weights or stakes, to force the main stems to grow parallel with the ground.
This treatment can be continued right up until flowering (though it will take less effort later on to keep a plant trained horizontal if the process is started early). It’s not a bad idea to let the last quarter of the main stem to return to growing vertically around the end of summer, as this should improve the quality of the large main cola produced at the top.
The branches produced by a horizontal main stem will usually grow straight up, like individual plants, and even a plant with a 3m main stem can be kept below the line of a fence.
If the wall is only 5ft/150cm high, it’s probably better to keep the horizontal main stem very close to the ground, as even the upward-growing branches could peek over the wall with a strong Sativa growth pattern.
Pruning plants during vegetation is another way to reduce their height and make them more bushy (as every stem and branch that’s cut will split into two small ones). In the case of a Sativa, the pruned plant will probably still have to be trained or tied down.