What's a good and consistent method to answer the non-market sizing questions such as: how many golf balls fit into X; how much does an airplane weight, i.e. questions of that type? Thanks and cheers

First, understand what is the "ocean" (e.g. assess the main dimensions of plane, etc.)

Second, understand the unit with which you deal (e.g. ball, ppl, table)

Third, understand the limitations & bottle-necks (e.g. has to be a space for the people to move back & force)

The last, do the math

Important: don't try to be precise. The way you think is much more important than the figures. But don't diminish meaning of figures as well.