As RWF stated, your sizing should be based somewhat on your comfort level with your excessive flows. If you undersize it you might exceed its capacity and cause an SSO. If you oversize it you waste some construction cost and possible operating costs (excessive aeration/mixing).
Depending on how long you hold the waste and it's source (I&I, surges from industrial users, etc.) you might or might not need to aerate it.
Tanks have been used for storage. What will the baffling do? If it is for aeration, mixing, or other primary treatment ok, if not, do you really need it? Make the bottom where the tank will drain. If it doesn't, you will have odors. Make sure your hydraulics work.
One of my main concerns would be the regulating authority. What will they allow?