The species F magellanica will grow to 10 ft but is very amenable to pruning. I am not familiar with the cultivar Cape Horn.

I suspect it is not a true magellanica, as there are very few named forms of that species, but rather one of the many hybrids that are sold as 'hardy' fuchsias but with a erroneous use of the species name as well.

I am not familiar with this cultivar and it is not listed with our Hardy Fuchsia Society data base. But online photos - if accurate - do seem to indicate it will get taller than 24" and wider than 36". Maybe as much as double that size. And you can prune hard and often should. Hardy fuchsias can generate a lot of skinny, twiggy interior growth that should be cleaned out periodically. One collector once advised that anything skinner than a pencil should be removed in spring before leaf out. And of course you want to remove any deadwood.

I have a number of different hardy fuchsias in my garden and I am not very methodical about pruning. I do attempt to clean all of them up each spring but I only prune hard when I feel the need to reduce size. I just cut back one five footer to about 10 inches. It will be back up to size in no time!

Because I am pretty sure yours is a hybrid, it is likely it will stay more compact than the straight magellanica species. Most hybrids do. But they also tend to be not as hardy as the straight species. I'm not sure I'd feel comfortable growing one below zone 7. Even in my very mild climate, some of the hybrids are very slow to respond from a sharp winter.

Thanks for the background and context. And for answering my clematis question of a few days ago. I'm trying new plants which means I'm in dangerous waters (for me), especially when there so many related (or only seemingly related) choices.

If I prune this probable hybrid between seasons (or even cut it back most of the way?) and snip it back some during the summer if it is really jumping up, is there a good chance I can keep it to the 2'?

If it makes a difference, I'm in zone 7a, so actually I guess it's basically going to die back to the ground each year, assuming it survives at all. I wonder if that controls for size?

I've only grown one fuchsia before, Gartenmeister Bonstedt, which behaves more or less like a bedding plant. It's maybe my favorite plant ever. But I don't think I've ever seen these other types in person. And even Gartenmeister is something I only discovered because this one local family-run place orders them each year.

Because this could be of marginal hardiness for you - so pretty much acting as a dieback perennial - that should keep size in check. If you do need to prune, wait until you see signs of budding in mid to late spring and cut back then. These shrubs can be late to leaf out and one can be fooled into thinking they are dead, only to be surprised by some new growth appearing at the base and slowly working its way up any live wood.