Factor into the decision what will be easy to clean.

I agree with you that 48" is large for a powder room (and one that large isn't necessary), but that would look best. If you go with a 36" you should have the same amount of space between the toilet and the wall and between the toilet and the vanity.

Is this your guest/formal powder room?

What do you like to put out for guests on the counter? Paper hand towels? Tissues?

My wall is also 7 ft and my vanity is 39" wide with a 41" countertop.

Just enough room to have a single handle faucet, liquid soap bottle, a square tissue box on one side and a rectangular paper hand-towel holder on the other. I wouldn't want it any smaller.

I also have hand towels on a ring, but many prefer to use the pretty paper towels, specially with a larger gathering. Also have a glass shelf over the toilet with a faux-orchid arrangement (no window, so real plants won't survive!).

For a powder room, I'd go even smaller and then put a tall linen cabinet next to it where you can have closed storage. Assuming this doesn't serve a guest bedroom and is really just a powder room, no one really needs to store anything in the vanity, but you might want space to store linens, extra toilet paper, household supplies, etc. I'd split it 36" for toilet / 24" for vanity / 24" for linen cabinet (or you could do 30" / 18").

What about allocating 42inch to each the vanity and the toilet? That wouldn't feel too big for the vanity (especially if you get a floating vanity) and you would still gain extra storage space. Or if you decide to maximize the space, with a 48" vanity you can get a taller narrower mirror or a round mirror and add wall sconces on either side to balance out the heft of the cabinet. A 36" vanity would be more affordable and would still look great though. I would nix the storage tower and opt for a larger vanity if you need the storage space. I think budget and storage needs will help you make the final decision.

I like the 36 inch space for the toilet and I would put a 42 inch stand alone vanity 6 inches from the wall on the right. Two drawers in the 42 inch vanity would be nice for towels, extra soap.

Our previous home's powder room vanity was like @chispa's with a 42" top. I appreciated the extra storage under the sink for hand towels, paper guest towels, soap, toilet paper, Lysol wipes, etc. The surface was outfitted the same way. In our temporary home where we lived while building, the powder room had a 36" vanity and that felt too tight.