Make sure you buy a window that is certified as an "egress window". The size is for the open space with the casement(if it is that style) fully opened. Cheating by removing the casement with any tools is not allowed when there is a fire. Just because it seems to have a nice large opening is not enough, because it must be installed low enough for children egress and large enough for a fire-fighter with gear (including tanks) to fight the fire from the inside.
The height of the bottom of the opening (not always sill or rough opening) above the finished floor will probably be 42" or 44", depending on your locality, so call and check that.
Make sure the window is certified and installed at the proper height and you should be OK