8hp engine with a 16 gpm 2-stage pump will be fine .
The size of the reservoir depends on the shape of the tank. The greater the distance between supply and return the less capacity needed. Timberwolf TP-2 has a 16 gpm pump with a 7.5 gallon reservoir which is typical for splitters that use a combination tank and axle. A tank that is a square cube are much less efficient. The more compact the tank the more capacity needed, sometimes as much as double.