new to cruising,
there are no silly questions - just silly answers from time to time.
You are certainly not too young to go on a cruise. Actually, I am in my late 20s, and I have been taking cruises for many years.
The "smaller" ships in the North American mainstream market today are approx. 70,000 gross tons (that's more than 1.5 times the size of the infamous "Titanic"!), and accommodate between 1,500 and 2,000 passengers. The really small ships are usually in the premium or luxury segments of the market.
Like Marita said, Royal Caribbean itself offers different kinds of ships. Their "Voyager"-class ships offer all the unique features Marita mentioned, and "Freedom of the Seas", which is now under construction in Finland, will enter service next year with even more things to do (including an onboard surf park). The "Voyager"- and "Freedom"-class vessels mostly operate one-week cruises in the Caribbean.
I agree with Marita that you should get in touch with a travel agent specialized in cruises - not instead of asking your questions here, but in addition to doing so.