Thermoelectric conversion (TEC) technologies, which convert heat into electricity, have received a great attention, because they are expected to be a powerful approach to utilize wasted thermal energy. Here we present novel thermoelectric converters based on the spin Seebeck effect (SSE), and show their scaling law which is largely different from that of conventional TEC devices. We experimentally demonstrate that the TEC output signals straightforwardly increase with the size of the converters. This scaling law enables us to implement simple-structured thermoelectric converters by using productive film-coating methods. Such coating-based TEC techniques may pave the way for a wide range of applications using a variety of heat sources.