1. Your body temperature is a little higher than usual.
2. Your body temperature is a little higher than your temperature is usual.
(Does #1 mean #2? Is #2 the full form of #1?)
3. He is as tall as I.
4. He is as tall as me.
(Are both OK? Which one is commonly used? Is #3 for written expressions? Is #4 for casual expressions?)
5. He is not so tall as me.
6. he is not as tall as me.
(Are both OK? Which one is commonly used? Is #5 for written expressions? Is #6 for casual expressions?)
1 is better than 2 (too wordy!).
3 is correct; 4 is incorrect. I do hear people use the construction in 4, but it's usually a child or teen or someone not well educated.
Neither 5 nor 6 is correct. It should be this:
He is not as tall as I.
(or He is not as tall as I am.)