You are correct. A 10' breastshot will have much more torque than a 5' overshot. But be aware that the bigger wheel will spin much slower, and have about 3 times the volume and weight of materials, also requiring bigger bearings.
You can also construct a flume to elevate the water feed to an overshot by a couple feet (of course depending on the shape of your property terrain)
You can increase the torque of the overshot by making it wider, so you have many options, depending on the torque/RPM's that would be useful to your application.
Its usually a given assumption that because of the cost of materials, it is desireable to make a small high-RPM alternator. However, if high RPM's are not a viable option in your situation, its the magnet speed that does the trick. There is a post somewhere here from "Cameroon" where a water wheel turned a very large diameter/low-RPM alternator. Although the shaft RPM was relatively slow, the rim speed was high, bearing in mind that the load resistance (charging a battery) also has more leverage to push back against the alternator shaft.
You may find it useful to run AC from your water-wheel/alt to the battery, as AC doesnt degrade as much as DC over a distance. Then put the AC/DC rectifyer by the battery, which should be near the building the elctricity is going to be used in, to shorten the distance the DC travels.
Whatever you do, please post some pics and a write up of how it turned out. Good luck!