Learn Turkish: Sen uzun musun?

It follows 4-way vowel harmony. The question particle changes its vowel to match the last vowel of the word right before it. Since the last vowel in uzun is u, the particle must become mu.

Why is the personal ending -sun attached to the question particle to make musun?

In Turkish yes/no questions, personal endings for "I am", "you are", or "we are" always attach directly to the question particle, not to the adjective. That is why it is uzun musun? and never uzunsun mu?.

I thought uzun meant "long". Can it mean "tall" too?

Yes! Turkish uses uzun for both "long" (like a road) and "tall" (like a building or a person). When talking about a person's height, you might also hear uzun boylu (literally "tall statured"), but just saying uzun is very common and completely correct.

Do I have to use the word Sen, or can I just say Uzun musun?

You can absolutely drop it. Because the ending -sun on the question particle already tells us exactly who you are asking (you), saying Uzun musun? is a complete and natural sentence. Adding Sen just puts a little extra emphasis on the "you".