Why is it that "daha" is used here but in the similar sentence "Sen benden büyük müsün?" it is not? Is it optional?
it is optional
What if I write "Benden fil daha büyük"? What will it translate to?
nothing, it is just a wrong word order.
Could "büyük" mean "older" here as well (as with people)? If not, what would you use instead?
As I pointed out elsewhere, this English is not the best, the sentence should be "The elephant is bigger than I [am]" rather than 'me', despite the increasingly accepted use of 'me' in the US.