Installing eco smart 27 with sub panel breaker box
You size the sub panel feeders to the size of the over current protection you are using to protect your subpanel. For example if you are installing a 60A supanel then you would install a 60A breaker in the main panel. Then using NEC Table 310.15(B)(16) you would find that a #6 cu in the second column or a #4 AL in the fifth column is capable of carrying 65A so you would use those conductors.

Note that your Eco smart 27 can actually carry up to about 113A. Since it is SMART it probably only allows a certain amount of current depending on your flow, but it is not SMART enough to know how much current is being used by the remaining loads in your dwelling. It could overload the service.

Rather than feed through your panel causing unnecessary strain on your electrical system.  I would recommend you install a separate enclosed breaker by you main disconnect and feed you water heater from it. You are allowed to have multiple main disconnects per the NEC 230.71. 

Finally I would check with someone and make sure your service is large enough to provide the adequate power for your new load. It could be your AHJ, utility company or a trained professional.

Hope this helps and good luck.