I can report that a 28" main wiper actually works fine with the stock 20" right-side wiper in place. If an arm was bent or off a notch, they'd probably collide, but this setup provides the greatest swept area. I'll see how close they come under a snowy load, etc.. The bottom end of the right wiper comes within 1" of getting snagged by the extra long main wiper as the big one starts uphill.... I used a 28" Bosh beam-style wiper, which has the lowest aero profile and seems to still handle the curves just fine.
2012 i-SE "MR BEAN" 94,000 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
1983 Mazda RX-7 EV 43 kW 10 kWh
1971 "Karmann Eclectric" EV 240 kW 19 kWh
1965 Karmann Ghia Cabriolet