What is the difference between "velký" and "veliký"? The dictionary at slovnik.seznam.cz didn't find the second one.
There is no difference. At least not in the meaning although people might prefer one or the other in some contexts, you can find it in other related threads, but I wouldn't care now anyway. Treat tham as the same.
So, if I am asking this ,should it be Jsem velka?
If you are female, then yes.
Can this be "Am I fat?"
Hi. Why "Jsem vysoké" is an incorrect answer?
Vysoký rather means "tall". Also, note that vysokÉ is the neuter gender form.
If i say "nejsem veliky?", does it mean the same with "jsem veliky?"
No. In English "Am I" and "Am I not" also do not mean the same.