Since there is no "e" or "hi" in this sentence does that mean it should always be translated as "it is huge" or could it also mean "he is huge" or "she is huge"?
Normally translate it as 'it', unless the context makes it obvious that 'he' or 'she' is meant.
Is this prefix limited to mawr or is it productively used on other adjectives as well? For example enfach, enddrud, endal…?
You will see en- as a prefix on quite a few words, but it is not something that you can add at will. Sometimes it is not a prefix at all, but an integral part of the word. Have a browse in a dictionary for some examples of both.
When is it ok to completely remove the pronoun like this? To me it looks like "is hugh"... Would "Mae o'n enfawr" and "Mae hi'n enfawr" also be ok?