I'd suggest 21's to go in the ground, or 36's to grow on in pots before planting in the ground. Even with TLC, you'll have trouble adequately watering small plugs on that kind of slope. Get the root systems established, and they'll deal with the issue more efficiently. Panicum is a good choice, because most of them are sod-forming and will hold the slope together nicely. Species P. virgatum and the 'Strictum' cultivar have grown well for me from seed, and you can create large clumps quickly. My favorites in the colored cultivars so far are 'Shenandoah' and P. amarum 'Dewey Blue.' 'Northwind' is very fast growing and upright.