Thanks for the quick reply!
Unfortunately, I wasn't sure what you meant by the interface of the stop and I hope I understood what you meant by underside of the stool. Here's what I've done. I apologize in advance if I misunderstood your comment.
what i've done in the meantime is removed the trim that covers up the edges of the window. I'll attach a picture here.
Then I measured from edge of the window frame to the other edge of the window frame (where the arrows are). I did that in three places, and the shortest distance (for width) was 51.5 inches.
For height, I took the measurement from the top of the window frame to the highest point on the sill against the back of the stool. the shortest distance was 51 7/8 ".
Please let me know if that makes sense.
If so, would you also mind letting me know if I should reduce by 1/8 or 1/4 inch these measurements when ordering.
Thank you.