In my experience, the email usually says which hotel you are in. But you still have to go to the transit accommodation desk on arrival to collect the voucher, so I suppose the hotel could change...

I had an stpc recently but can't remember how much the meal voucher was for (250 riyals rings a bell). I was at Oryx Rotana and didn't use the full amount, but I think you just charge everything to your room and settle up with the voucher and any extra when you check out. Other hotels may vary...

I don't know whether the voucher amount may vary depending on the length of your stopover, but remember you will only get a meal voucher if your transit is longer than a certain time (11 hours I think?)