How to set an experiment in which i need to find volume of one water drop?

Microscale could be best bet as evaboration and other factors can come in play. otherwise contact angle measurements could have something related calculated via surface tension.

something that is possible, calculating mols via gravimetry.
finding the suitable material could be tricky as if it needs to be instant. silica gel doesn't necessarily work

adsorption vs. absorption

Hm, is this from a book you've read? I don't recall studying this, so I'm not sure how to answer.