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Figure the spacing between your vertical posts. and the vertical height between your existing sill and header.
That should tell you what size windows will fit.
Purchase windows that DO NOT have an exterior nailing flange. if all you can find are windows with a flange make sure they are the kind that snap back or can be removed.
With a nailing flange, you will have to clad the exterior to cover the flange...an extra step perhaps.
The windows can be installed by screwing them in through the side styleinto your existing verts. Precaulk the opening, slide the window in, screw it in place (assuming square & level) caulk the exterior, cover caulk joint with some moulding (quarter round, inside cove moulding, etc.)
Wood, vinyl, fiberglass, etc. is your choice.