if you have no idea and haven't been on skis for awhile, just put yourself into the hands of the shop, talk to them, and let them help you out. Look on internet and find shop that is close to you that looks like they do a lot of business. If they are thriving, that means they turn over a lot of skis (so will usually have newer gear). If they are a dying shop they can't afford to get newer skis.
1) Spend more time worrying about your rental boots and making sure you get boots that fits you well. This both means you aren't cutting off circulation and your foot is falling asleep, but also means there's not a lot of empty space and your foot is just swimming around.
2) The ski should be chin to forehead height, (beg-advanced), If it is twin tip or has a lot of splay you may want to lean towards the longer length.
3) They should look "fairly" new, and definitely not tooo old and beat up.
If you picked a shop well and explain to them your background they should pick a good ski for you or just have a fleet. You may want to just rent on the mountain.
4) Reserve online first, go early esp. if you're doing on-mountain rental.
5) they may just put you on their "intermediate" performance ski which is fine for your first day in 13years.
Don't worry about the rest if you picked the shop right.