there are commands screen width() and screen height()
from this you can determine the current width and height and use them as parameters in your box size.
eg: ink rgb(255,255,0),1 : box 0,0,screen width(),screen height()
Did you mean something else a little more complex?
after you have set the display mode store the current resolution into a couple of arrays or global variables.
eg:
global SW : SW = Screen Width()
global SH : SH = Screen Height()
or
eg:
rem ignoring the zero location in the array for simplicity
Dim SW(1) : SW(1)= ScreenWidth()
Dim SH(1) : SH(1)= ScreenHeight()