Replacement window inserts are a fully assembled window in what's referred to as a "secondary frame." It slips into the existing window opening and you fasten it to the existing window jambs. You'll lose a little glass area because the "secondary frame" takes up space that used to be taken up by the glass. You don't have to pay a contractor to come out and measure your windows before you order replacement inserts. You can do it yourself.
Measure inside width of old window frame. Measure from jamb to jamb. The jamb is the slightly recessed strip of wood right next to the part you raise to open your window. Take three measurements: one at the top of the window, one at the middle and one at the bottom. Use the smallest of these three measurements.
Measure the inside height of old window frame. Measure from the top of the window sill to bottom of the top jamb. Again, measure in three places: at the far left-hand side of the window, in the centre and at the far right-hand side. Use the smallest of these three measurements.
Check to see if the window frame is square. Measure the window diagonally from corner to corner, jamb to jamb. These two measurements should be the same or within 1/4 inch of each other. If it is larger than that, a replacement insert will not work and you'll need to install an entire full-frame replacement window.
Measure every window, even if they appear to be the same size so that all your replacement window inserts fit properly. Measure your windows from inside your home.