I'm planning to add this to one of my games, but I'm wondering how I can have it so that it only fills up the view, and not the whole room?
Look at your textbox code and find the user variables section. Replace variable destinations as you see below. Depending on the size you have your views set to, you may need to adjust the variables "y1" and "y2".
// USER VARIABLES /////////////////////////
x1 = view_xport; // x1 :: Left x position of textbox Adjusts textbox to left.
y1 = view_yport+360; // y1 :: Top-left y position of textbox Adjusts the top.
x2 = view_wport; // x2 :: Right x position of textbox Adjusts textbox to right.
y2 = view_hport-1; // y2 :: Bottom y position of textbox Adjusts the bottom.
Hope this helps. :D~