Smaller tanks are more of a challenge to keep due to the limited water volume. When parameters go south, they go quickly.
I have a 4g all the way up to a 125g and I find I spend more time on my 4g pico than I do on the 125g!
55g is a great startup size in my opinion, if you must go smaller I found that my first sw
tank, a 29g was a good place to start as well. If you are tight on space that is. The 29g is 30" long while a 55g is 48" long.
Hope that helped.