Section 1.5 of Biological Physics by Nelson lists
1.5.1 Molecules are small
He describes how in 1773, Benjamin Franklin estimated the linear size of a single molecule from observing that one teaspoon of oil covered half an acre of pond!
1.5.2 Molecules are particular spatial arrangements of atoms
1.5.3 Molecules have well-defined internal energies
1.5.4 Low-density gases obey a universal law
Boltzmann's constant is universal, i.e., it does not depend on the chemical or structural details of the system being described.
At room temperature
k_B T = 4.1 pN nm (most important formula in this book!)
I tend to think k_B T = 25 meV since this is a useful scale in solid state physics.
But, Nelson deliberately writes it terms of a force scale (picoNewton) and length scale (nanometer) that is relevant to the physics of cells.