The C22
The 1987 Manual for the C22 states:
"4.5 Auxiliary Power
4.5.1 Recommended Outboard Engine
An outboard engine of 4 to 6 horsepower should be adequate to propel the Catalina 22 at hull speed under usual conditions.
A larger engine will not increase hull speed and may add additional unnecessary weight in the stern.
Long shaft engines are preferable, however, standard length shaft engines should be adequate for most conditions."
The 1977 Manual states:
"OUTBOARD ENGINES
The Catalina 22 requires an outboard engine in the 3 to 6 horsepower range should you desire to purchase one. The factory provides an optional outboard bracket or you may purchase a commercial one. "Long shaft engines" are preferable but regular shaft will work satisfactory. "
I would add in agreement with what has been said that there are situations where one may want to have more Hp/speed if one is in high currents, winds or waves BUT this can present a problem with weight. Especially if you are considering a 4 cycle outboard engine where the weight is about 3/2 of a 2 cycle of the same HP. More weight will slow the boat and make it stern heavy while it may not necessarily get the boat moving faster than hull speed. So adding HP may only be adding weight and gasoline cost. I would probably not advise anything over 8 hp. but your situation may be different. Some sailors with C22s even use electric trolling motors with success.
We sail on a lake and our Yamaha 6Hp electric start. and it gets us along at close to 6kts on a flat lake with no head winds. Heading into 3ft waves it will go somewhat slower but that situation is rare and I may consider motor-sailing if that is the case. Typically, I have my 6hp engine's throttle set at about 25% and virtually always at less than 50%.