As long as it does not overflow when the power goes out then yes, technically it's big enough. That being said, the larger the volume of your sump the more room for equipment like a skimmer, reactors, heaters, spare live rock, etc you will have, and the larger total volume will mean more stability in overall parameters.
I use a 100g Rubbermaid stock tank for my 60g frag tank - it has all the live rock that provides the biological filtration for the system. For my 120DT I use a 40 breeder and think it could be larger (if I had room under the stand it would be).
hth