With these boats you can use pretty well any size pack so long as the voltage is appropriate for the winch and rudder servos.
Generally speaking 4 cell alkaline will give you 6 volts which is commonly used . you can use 5 cells NiMh batteries which wil a lso provide 6 volts. Using a 4 cell NiMh configuration will only supply 4.6 volts, but that’s OK as well, just less grunt.
For most of us the standard 4 cell plastic holder is most common and will take either AA chemistry batteries (but not a mixture of).
I use 6 volts because these boats place a fair amount of demand on the battery at times to operate the servos, and while we aren’t in the sport of racing these boats there is no reason why the sails and rudder can’t operate a bit faster if you want.
While Gary gives full instruction on accommodating the Rx (radio receiver) and the battery it can be up to you where you place them. On my Emma I do this at the top of the keel box and use sticky Velcro to keep it there. You can add a switch if you wish. Since these hulls are steadfastly very dry you should just place the switch for easy access.
Bob W says Its worthwhile giving all the electrical contacts a spraying with corrosion x. This will inhibit any problems with water on electrics. I first discovered corrosion x when I built a waterplane.