If that was my yard, I would run a 1" conduit to a spot about ten feet down the wall (for a future sub panel) and then a 3/4" conduit from there to the back wall of the 'box.' Run the 1" so the sub panel can sit back from the wall a couple of feet so it will be easy to reach, but still easy to hide with a plant or two.
Also, figure on what type of cool lighting you want for the hill and yard, and make plans to run an extra circuit out of the sub panel now. If you don't put huge spotlight bulbs or lots of power-hungry lights, you could simply put it on a photo cell. If you want dazzling lights, plan on a set of switches/dimmers, or a timeclock. Easiest? Just add a remote switch later. If you put in low-voltage lights, most come with a built-in timer. They are a pain - unless you go with pro-quality stuff. Some LV systems have a photo cell and a simple switch: they will come on at dusk and automatically shut off 4, 6, or 8 hours later. Those are the ones to get: no messing with the stupid timeclocks all year long as sunset times change or power failures happen.
Thinking of Fountains or other things? Make room for the power to run those as well. If you never put them in, you still won't be sorry you put in a few bucks' worth of PVC conduit.
And as to size: the largest HotSpring is the Grandee/Vista - 8'4" by 7'7" - add a couple inches on each side for a Cover Upright lift. That lift will put the lid up on end like the lid of a pizza box. It will mount on either end of the spa.
Or - just allow about a nine or ten foot square to fit most any spa.