Is there a way to determine what engine you need to use, based on wing area and weight? Like a formula. I think this question may have been asked before. Thanks.

There are a couple computer programs to do the calculations you want. One of the programs is called 'Motocalc' and has been around for a while. You can check out the details at their website:
www.motocalc.com. You can download the program for a 30 day trail and after that the cost is $35.00. That will give you motor size, battery capacity, prop size and expected run time and a whole lot more info.

There is another program available but I can't recall the name of it. If this is a one time occasion give Kirk a call at New Creations Hobby and he can run it on 'motocalc' for you, if you talk to him nice. The idea is for him to sell you the "go" stuff too. Good people that will treat you right. Go to their website: www.newcreations-rc.com
You can get a phone number there.

There may be other calculators I'm not familiar with.

Good Luck.