Remove the casings (mouldings) that encircle the windows. You are mistaken if you assume you can weigh the window without removing the casing. You’re completely unaware of what the previous installer did. It’s possible that the current window is either too big and pushed in, or it’s too small. I know of a young renovator who was tasked with installing windows that his father had ordered. Since Dad didn’t take accurate measurements, all of the windows were too wide for the rough openings. He was unable to return them. He might have just said no if it hadn’t been his father. That work makes me shiver just thinking about it. Make sure the rough opening is square by measuring the diagonals. The new windows should be around 1 inch smaller than the rough opening in both dimensions. Click more info here.
This will give you about 12″ for shims all the way around the window. You will want to make the windows a little smaller if the openings are not square, as calculated by measuring the diagonals. If the wall is 2×6 or 2×4, the supplier may like to see. You’ll also need to determine whether or not you want “brick moulding” on the exterior. You’ll need to determine the size of the brick moulding and equate it to what the manufacturer offers, then change accordingly. Order the windows without a nailing fin if you want my opinion.
Examine the new windows as they arrive. Next, test the new windows for damage and to ensure that they are the correct size. You don’t want to discover this after you’ve taken down the old window. The aim is to remove the old windows while causing the least amount of harm to the house as possible. That means you’ll have to put the sledge hammer away for the time being. Remove some caulking so it won’t be a concern, which it might be.