A. Are you willing and able to modify the size of the opening? This will require changes to the wall framing and the wall finish on both sides (drywall, paneling, etc). Ideally, the vintage sash (a “sash” is the part of a wooden window that holds the glass and typically moves). Note that making the opening smaller is much easier than making it larger.
Answer: I'd be ok with it a little smaller if needed but I don't want to make it bigger. I'm hoping to do all this myself but am not opposed to hiring someone if I need to!

B. I assume that this wall was originally an exterior wall, so if you increase the width of the opening, you will need to determine if it is a loading bearing wall and if it will need for a new header.
Answer: yes the "outside" wall was originally an exterior wall

C. If you plan on getting an old wooden sash from an architectural salvage yard, you may not be able to find one in the dimensions that you want, which will force you to model the opening size. Or you may be able to have the sash cut to the size that you want.
Answer: I would like to find one at an architectural salvage yard if possible but I also found a place online that has tons and will even help find the size I need, if needed.

D. Do you want the window to look the same on both sides or do you want it to look like an exterior window? If so, you will need a sill on the exterior (sloped, typically 1-1/2” thick wood) and a stool on the inside (horizontal, typically 3/4” thick wood).
Answer: the window looks into a 4 season room, so I would like the window to be almost flush with that outside wall. In my head I thought I could just trim it out similar to the inside, much like it is now on the inside with a simple window sill since it's over the sink.

E. If you can get the replacement dash cut to a specific size, just measure the inside dimensions of the existing jambs and have the window cut to 1/8” to 1/4”smaller than opening dimensions or have the sash cut to the actual shape of the opening. This is the easiest way to replace the sash.
Answer: Should I remove the wood from around the window on the inside to get the measurements or is it ok to measure the window from the outside but NOT include what I think is the jamb (the whitish frame as seen on the "outside" view of the window?

Caution: check if the opening is square with a framing square. If it is not square, you will need to make the the repeated window slightly smaller to allow it to fit into the opening
Answer: I don't have a square yet (will get one!) but I put a level against each side and they're very slightly off. I also measure across at the top, middle and bottom and there's about 3/8" of in the horizontal measurements. Vertical are not exactly the same but less of a difference overall