I would start with 8x10 as the smaller 5x7 could be tricky to get dodging and burning, important skills for any printer, you would easier see any mistakes and successes on the bigger paper, as for paper developer,dektol would work, but there are other choices, I use the fotospeed wt 10 warmtone paper developer, but possibly Ilford multigrade would be a better choice,. as for stop, you use the same dilution rate for film as for paper, and I mix up a litre at a time and use it for both film and paper,.