I used to use 4x5" full length. Maybe I just got lazy, but the past 10 years or so I just leave the bitz on 3x120, same twist angle and use 4" of uncut fletch. EZPZ, cut-em' and glue up like my regular arrows.
12" of full height feather works plenty good to parachute the arrow down where I can usually find it. If I need more drag I'll cut them a little longer, but so far 4" seems to work about right for not losing too many arrows on tree top squirrels, still shoot relatively flat and retain some thump at 25yds or so over the 4x5's I used to shoot. Rarely have to mess with or tweak for spine with 3x4's too.
Anyway, 3x4's have worked out very well for me, no setup changes to make and sufficient drag to keep the arrow from reaching escape velocity.