When measuring existing wood windows, measure the width by measuring between the interior edge of the decorative moulding at the top, center & bottom of the window. Measure the height from the interior edge of the top decorative moulding to the bottom edge of the interior sill, at the left side, the right side, and in the center.

In each direction always use the smallest measurement  for determining your opening size.

When measuring existing aluminum or vinyl windows in finished drywall openings, measure the width from drywall return to drywall return at the top, center & bottom of the opening. Also measure the height from drywall head to drywall sill at the left side, the right side, and in the center.