Originally Posted by Retro Grouch

That's not much information for the question you are asking.

It's all a matter of simple physics. How much power can you produce, how much weight are you trying to pull up the hill, and how fast do you want to go down the other side? Different variable inputs = different answers.

In all honesty, I think you've got to try it with whatever you've got. If you think the hill is too hard, figure out what you have to do to acquire an easier gear.
Two things to remember:
1. If you put a Ferrari transmission into a Yugo, it still won't go 150 MPH. You've got to have enough power to get the job done. On a bike or trike, that comes from the rider.
2. Everything on a bike (or a trike) works together. You can almost never change just one part. If you install a cassette with a 40t big cog, you are going to need a longer chain and probably a derailleur change to make it work.