I also like yeastcalc.com. I remember hearing from a webcast from someone from wyeast that for a hefeweizen, pitch 6-12 million cells/mL, depending on what you want to accentuate. Since you are shooting for more banana, then closer to 6 mil/mL would be good. According to yeastcalc.com, a 1 liter starter with no aeration (not even intermittent shaking) will give you about 7.4 if the viability of the pack is high (97%), so 1 liter might work great for you. When I brewed a dunkelweizen last month with this yeast, I used a 1L starter on a stir plate so I could freeze some of the yeast and still get about 9 million/ml. I think I ended up pitching a little higher, at like 10 or 11 million/mL, and I can definitely taste and smell the banana, but it is a little more subdued than I'm sure you'd want.