Voltages and charges with regard to ESD
The relation between charge Q, voltage V, and capacitance C is Q = V*C.

If something has high voltage, and low charge, then it has low capacitance.

The capacity of the human body with respect to ground is often quoted as 100pF. This is an approximate, ballpark, figure.

When you calculate the capacitance of two conductors in a given geometry from first principles, and you have circular or spherical symmetry, for instance a human body which looks a bit like a ball, above a ground that looks like an infinite half plane, then the capacitance tends to vary as the logarithm of the ratio of conductor diameters. That is, it varies very slowly. This means you can change the diameter of the body, or the distance above the half plane, by orders of magnitude, and the capacitance changes by only factors of two. For things about the size of a human, and typical spacings to the ground and grounded objects, the capacitance comes out within a factor of 2 or 3 to 100pF.

The way you would measure the capacitance of a human body with respect to ground is to charge it up to a given voltage, then see how much charge flows to ground when you discharge it. C=Q/V.

Just in case you're wondering ... If the capacitance of a human body with respect to ground depends somewhat on the distance above ground, then what happens when a charged human climbs onto a chair? Glad you asked. If the amount of charge on them stays the same, then as their capacitance varies, their voltage varies inversely. In fact there are electrical machines that use this constant charge variable capacitance principle to generate high voltages, the electrophorus, Wimshurst and Van Der Graff generators for instance. Wikipedia will give you details on all three.