Starting small and expanding is a good way. You'll want to want to
match your inverter to your loads or you'll have problems. My first
inverter was a 1000 W and was way too big for running a few lights,
the computer and the radio. It had a cooling fan that was always on,
noisy and drew my battery down. Turned out a $30 Walmart 175W box was
a better match for what I was running. Then when I started adding
things with motors like the refrigerator the 1000W wasn't big enough!
Now I run a 1800W for big loads and the old 175W for the small stuff.
It all depends on what you're wanting to run, but I'd suggest a small
inverter and a Kill-A-Watt meter to measure your loads as a good place
to start. Once you've measured your loads you'll have a whole lot
better idea of how much inverter you'll want.