So you’re wanting to mount outside, on the wall? If so, then you want it to be slightly wider. One to two inches wider, total (1/2 to 1” each side). If you have a sill then you want it as wide as that, which is probably two inches wider.

You could make it even with the frame but if you’re using fabric, it may shrink slightly over time.

Thanks. I actually prefer not to drill into the wall as our walls are plaster.

On the frame is not a good place. You can’t do an inside mount?