Measure between the brick at several places to get the (brick to brick) maximum width opening. Now measure from the brick sill up to under the steel or brick lintel at the top, this is your brick to brick maximum height opening. Your new window cannot be any larger than either of these dimensions.
Now remove the inside trim on the window and measure between the stud opening for the width, and the sill to header opening for the height.
Take these measurements to the window store and either get the largest window available that will fit in the rough opening you measured from the inside and still not have an overall unit dimension larger than the brick measurements on the outside.
If you need to special order a window to fit closer to the brick openings or overall unit dimensions, order it about 1/4" less on width and height than your outside brick to brick dimensions. This allows you to caulk at the sides and top and bottom.
You may need extension jambs so the frame of the window will end up flush with the wall surface on the inside. You can make these extension jambs later so don't worry about ordering them with the window.
Most lumber yards (local) will be glad to come out and measure your windows for you (if you buy the windows from them).
Just remember the *unit dimension* is the overall largest dimension of the window frame. The * rough opening* is the size of the existing hole in your house now. Your wall thickness is the distance from the exterior wall sheathing (not including brick) to the inside wall surface. The standard thickness is 4+9/16".
Double hung windows raise up and down (both upper and lower sash)
Single hung windows only the lower sash raises.
Casement windows crank out, Awning windows open out at an horizontal angle so they can be partially open even if it rains.
Sliding windows slide side to side.
Be sure to ask if the windows meet the requirement for energy tax savings rebates.