Reservation means Cache Size... how much vFRC do you have? How much are you willing to assign as Cache to that vDisk? I am pretty sure that only a small part of your 2TB database is active right? This small part should fit into the Cache.
AND NOW FOR THE LAST TIME...... you calculate the Blocksize by SIMPLE MEASURING the current Block size of your vDisk and the best way i already told you.
Regards
Joerg