Stagecraft isn't house building, so you need some valid ideas from experts. Head for your library, check out a number of books on set building, etc. and scan through them, making a notebook of the how-to directions you find. If realism is important, sketch a set of real doors at a Pella store or similar, then scale up those dimensions to your stage doors. In the theater everything is a little larger than life. and some groups will standardize on easy dimensions for regular doors at typically 3' x
7' in a 4' x 12' flat. You probably won't use glass in your doors, but muntins can be glued in and nice sight lines milled in to them with a router not even bothering to make square corners. Door thickness in stage flats is usually by means of 3/4" stock glued and nailed to 1
1/2" wherever sight lines make it necessary. Glue, nails, gussets and braces are the stock in trade of the set builder. The activity is more fun than building houses because you get to see the results so quickly. The people who do the stage painting are often amazing at taking your product and giving it a life of it's own. Schedule a visit to a local school theater department if you can to look at old sets for ideas. Above all, keep it light, keep it sturdy. Have fun.
Joe