Currently, tuxmath runs either in fullscreen mode (preferred) or in a 640x480 window.  640x480 is pretty small these days for a game.  For this task, allow the user to specify a different size when launching tuxmath from the command line as an extra arg following "-w" or "--windowed", e.g.:

tuxmath --windowed 1024x768

Look in src/setup.c around lines 522-30 and 709-740 for current code that has bearing on this task.  You will need to change the behavior of win_res_x and win_res_y, which are currently constants.

Ideally, have the program remember the window size when it is run again.