The Fermi process is all about making rough estimates to get an order of magnitude type of result. So, for example:
Size of bathtub: ~ 2m x 1/2m *1/2m Vbath ~ 1/2m3 → 5*105 cm3
Size of brick: ~ 20cm x 10cm x 5cm Vbrick ~ 103 cm3
Number of bricks in bath: Vbath/Vbrick = 5*105/103 = 500
Note: I simply estimated (i.e. guessed!) the sizes above; I didn't measure anything. Often a way of "estimating" is to choose a value that is clearly too small, and one that is clearly too big, then choose the geometric mean as the estimate (though I didn't do that above). This often works well if many people make an estimate of something and you take the geometric mean of the largest and smallest of their guesses!