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.