At some point in any technology development, the developer needs to understand how big their market opportunity could be. This is especially relevant in disruptive technologies developed by start-ups or spin-offs as, due to their innovativeness can tackle different markets, but typically due to the company size, they cannot cope with all of them at the same time. The B.size tool is developed with such objective to help companies and/or researchers, with a guided process, to calculate roughly the order of magnitude of their business opportunities as well as a first estimate of their potential revenues in three different markets. The tool is structured into 4 sections to help users to think around the main concepts that can determine the size of a business opportunity and their capacity to take benefit of it. While some questions require actual figures, most of them can be answered qualitatively, in general selecting into a 0 to 5 dropdown field.