Measuring for replacement windows.
 

I have a brick house. Most of the advice I have read or viewed online recommends measuring for replacement windows from the inside of the window. I measure 32" across the existing windows jambs. When I removed the windows, framing, pulleys, etc. I measure the brick rough opening 36 1/4". Once past that 36 1/4" opening, there is actually 40" of space. Which width is better? The ACTUAL window width of the desired replacement windows would be 35 3/4". Thanks.

If you are measuring for new construction windows(not replacements), you need to measure the actual house framing opening by removing the trim and measuring to the studs. For replacement windows, you measure the opening of the old window frame the holds the sashes. Check for square by measuring diagonally corner to corner. Then measure in three places for width and three for height. Use the smallest dimension for your rough opening.

Measuring for replacement windows
 

I was measuring for replacement windows. I removed the old framing and pulleys in each size I wanted to measure. I gutted everything to the brick opening in the wall. The smallest brick opening on the outside was 36 1/4".

You're not measuring for replacement windows if, "I gutted everything to the brick opening in the wall." First off, you don't know what a replacement window is. That is a window that sits in the frame of the old window. Once you gut the window, you're looking at a ,"new construction " window.
You shouldn't pick up a tool until you know exactly what you're doing.
Try searching, "replacement windows" to get a handle on the subject and you see if your windows even qualify.
Ron

I've seen windows described as replacement windows if the do not have a nailing flange, and are instead intended to be secured to the framing solely through the window frame.

New construction windows use the flange that needs to be ancored to the sheeting behind the siding. The flangless ones can be installed without disturbing the siding. some new construction windows are designed to be converted to replacement windows. I'm thinking that would put more stress on the frame, so I wouldn't do it unless the manufacter specifically mentions it as an option.

to the OP's original question, you are looking for a window designed for a 36" wide RO, but it sounds like you might be able to go up to a 40" RO window (I think that would be a custom size) without messing with the structure, just the brick fascia. I'd stick to a standard size to save money and work in your shoes.

Measuring for replacement windows
 

Thanks for your feedback. I understand now.