Read the section on etymology.
There are three, each with slightly different meanings. The one you're probably looking for transliterates to "megalos" in English. If you enter great in the English to Greek box toward the right and click on Find, you'll see the results. The transliteration of each word is right below the Greek spelling.
3. There are several.
You'll need to decide which one you consider "great."
4. Lots of things.
Let us know what you decide.