Not a bad idea. Nowhere is it written that the curtains can't be significantly larger than the window. I once had a large mirror in the living room. I fixed it up with nice curtains, and was able to briefly fool some people into thinking it was a window.
On basic curtains, Big Lots can sometimes provide what you need. What size they have depends on what they were able to buy cheaply. If you find what you need, get all you need right then and there. They mostly buy close outs and remainders, so 'Here today, gone tomorrow'.