The best idea is to first build your steam plant and then plan your boat size accordingly. In small boats (under 70-80 cm) the steam plant's weight can really be a problem, especially if the boiler is made of copper.
Another thing to consider early on is the maximum diameter of propeller that fits your boat. The propeller size should be matched to the engine capacity. For example, I have often seen model steam engines coupled to propellers which were too small. This wastes a lot of steamfrom the boiler, whose power cant be transferred by the propeller to the water.
Do you already have an engine for your project?