Most benches aren't really made to sit on, unless it's going to be a steam shower. They're usually there for the ladies to hike up their legs for shaving, so they don't usually really need to be that big. One that's very popular doesn't even need to be framed out. Check the link below:

Here is a link that might be useful: Better Bench

I have the bigger better bench and it is perfect for sitting unlike the smaller one that is more for the leg shaving one leg up. Watch out too if you do the large rectangular bench at the end of the shower because then you will need to have the shower glass cut for it and it might be more expensive.

The thing I love most about the better bench Bill linked above is that it seems to float and there is nothing to stub your toe on. It seems like it takes up no space at all. We have a handheld on a slide bar on the side with the bench and you really can sit on the bench and just let the water wash over you-nice.

If the shower is large enough, I see no reason NOT to run a bench full width, and hang a hand held shower head above it: It's nice if you find yourself 'differently abled' due to a broken bone or bad back, and makes a comfy place to sit, prop up a foot, trim the toenails etc-- so it's good for men as well as women.

That said, I think it's better if they are on the tall side. Like a 'comfort height' [formerly ADA] toilet, a little extra height makes it easier to get up and down. Also, remember they slope of the pan will make the finished shower floor taller than the subfloor of the house: I forgot that when planning mine...

working with 45x60, I'd put it accross the 45 end, 16 inches out from the wall, full width, and about 20 inches above the floor so that it's about 18 when the pan is installed.

working with 45x60, I'd put it accross the 45 end, 16 inches out from the wall, full width, and about 20 inches above the floor so that it's about 18 when the pan is installed.

That's about right. I'll usually set it about an inch lower to account for the smaller stature of most women.

also, since you are barefoot, an inch lower puts it at about the same height as a chair height (17" or 18") with shoes on.
if it's 45" long, then most people will sit sideways on it, stretching out their legs across a portion of it. So a 16" depth is OK.

Use steel (or steel and wood) to support the span and you will get the floating in mid air effect of the Better Bench. Depending on your framing and construction capabilities, you can make other shapes.

To get it ready for tiling you have many more options Instead of filling it with a cement product (as the Better Bench does). One, use Wedi to cover it -- no need for plywood, and it cuts with a utility knife. Also, you could build it with plywood, screw 1/4" Hardi onto the plywood and then membrane it with Kerdi or Redgard or any other membrane. Also, since it's a small area, you could build it with plywood, and membrane the plywood directly, with Redgard, or with Kerdi and latex-additive thinset (there are procedures for thinsetting onto plywood, and on a small area the whole discussion about whether one ought not to thinset onto a wood substrate is less and less meaningful.) So, Wedi or plywood keeps it lightweight; no need to fill it with concrete.

HTH
-david