Normally, the system is set for 4 actors tops.
That meant that for a menu displaying the actors (top down fashion in a 480 pixel high screen), you'd have windows no taller than 120 pixels in height. In your case, you'd want them to be 96 pixels tall.
And for a battlestatus window displaying the battlers (left-right fashion in a 640 pixel wide screen), you'd have windows no wider than 160 pixels in height. In your case, you'd want them to be 128 pixels wide.
You need to adjust the Window_MenuStatus, the Window_Target (mostly the refresh def), and Window_BattleStatus (hint: actor_x = i * 160 + 4) plus a few more odds and ends.
You may want to edit Window_Savefile to reflect 5 members in your party as well...