Bob Bob wrote:
"In rough terms a half wave in free space on 2.4 GHz is calculated as
300/2400 MHz/2 metres or about 60 mm."

I agree with Bob Bob.

A quad antenna has a smaller maximum dimension then 1/2-wave. Its
circumference is a full wave, but that is divided by pi to calculate its
diameter. The configuration is not very important. It can be circular,
triangular, square or a figure with almost any number of sides.
Performance depends on the area enclosed by the figure. If square, it
has 1/4-wave sides. The diagonal is 1.414 times the length of a side. In
the case of a square quad for 2400 MHz, the sides are are about 1.25
inches long.

A quad with a circumference of 5 inches may seem small but it is not a
"small loop" which couples to the rest of the world via the plane of the
loop. The quad couples to the rest of the world broadside to itself.
Kraus gives the radiation resistance of a full-wave loop in Figure 7-10
on page 209 of his 3rd edition of "Antennas" as just over 100 ohms.

Best regards, Richard Harrison, KB5WZI