He he, that's a question for me.
He meant that he felt small, not proud of himself.
Lennon had first intended to say the correct sentence: "feeling two-foot tall" and made a mistake and sang "feeling two-foot small" but found it funny and decided to keep it.
(ha, incredible the amount of things you can learn with the Beatles!)
There is no such thing as "feets". "two-foot" is allowed when it is an ADVERB or ADJECTIVE. You can also use "two-feet tall". They are completely interchangeable! Not sure if one is "more correct" than another but both are used in real life and in the written and spoken media.