Some years ago, my brother's first boat was a 22' Santana. if I recall, he had a 3 HP motor on it and it did fine. I'm sure some here will tell you to have at least 5.
As far as the size of a boat goes, I could single hand my 34. it all depends on how it is equipped. Some will say larger boats are easier to handle than small ones, but you do need to learn how to sail first.
My first boat was a 30' Catalina and I had very little sailing experience before buying it.