Best is about 1 lb before molting, when the shell is full of sweet meat. After molting when the shell is soft you will be paying for shells that are half empty. There is already so much waste on lobster that you want the shells full.
I notice at my local supermarkets raw frozen lobster tails. I do wonder from whence they came? Lobster should always be alive when cooked so I suspect that the dead ones in the tank are being salvaged.