I had sex in January then got my period. It was enough to soak a tampon... Then had sex in February.. Missed the period after that it was just light spotting. My ultrasound at 16 weeks said my baby was 5 inch but apparently its supposed to be 4 1/2 inch at 16 Weeks so I'm worried I got Pregnant in Jan.

Dating is all Best Guess.
If you have a concern of paternity then a DNA test would be required to confirm parentage.

Why are you worried, Weeks pregnant is a guess bases on average sizes of the fetus.

Next if you have a very light period, only one tampon, then yes it could have been from January.
You will have to wait till baby is born and do a DNA test, only way to be sure.

This is a question for your doctor. All babies grow at different rates and based on what you said, its not likely you became pregnant in January. In a month's time your baby would have grown much more than 0.5".

There is a book called "My Pregnancy Week by Week". I suggest you pick up a copy. I think it will address a lot of your questions.