I would like to do what some modern games do and have a launcher that (for the sake of starting out) asks for a window size. How do I pass data from the results of this app to the game/main app?

First off, you prolly want a settings file for your program, so it can remember the settings between sessions. And in that case, your launcher can just write out new data to the settings file. (plain text is nice here so you can manual edit settings if something doesn't work right on someone's system they can just tweak the file)

Second, some programs "launcher" is not a separate program just a different window and is still part of the main application.

Ok, so I could use an .ini or .txt file (Or any format with text I choose). Noted.

So... I could make a child window and a drop down box.
Based on that drop down, I could return an int and choose a preset resolution from that?

I think that would work, but all I need there is knowledge of making a drop down.
Tut time.