hi there and welcome to YT. That is awesome that you are giving this little guy a chance he might not have had before. Yorkie standard is up to 7 lbs but there are alot of yorkies here that are bigger and we call them teapots here.
Yorkies are a little hard to potty train anyway but it isnt impossible. If he still marks you can always use a belly band of him. That is what I use on my males. I cant imagine how he got a chemical burn on his back but you never know these days what some people can resort to. Maybe since he was a stray he got into something he shouldnt have looking for food. Poor thing. Anyway I wish you the best and commend you for what you are doing. I can wait to see pictures of your big guy.