How can I make large windows look smaller?

You could try limiting the width and height to only the size of the window frame, therefore not having curtains beyond the top and sides. Don't use bright colors as that makes it a focal point and will make them look bigger, perhaps use curtains the same color as the walls to make them blend in and blend into the background and appear smaller.

Curtains on the inside and pull curtains to only show a portion of window/glass. On the outside, consider shutters or framing in windows.

Try using two rods and 2 panels per window, make sure the panels are wide so you can bunch (space)them to appear to be two windows and the rods place closer to the ceiling so they look taller and not wider and this give the allusion of a smaller window. I help this was a helpful tip😋