  Uncertainty relations for light pulses found in [Phys. Rev. A {\bf 86},
022118 (2012)] were derived in the three-dimensional case which emphasized the
localization in a volume. Here we derive the uncertainty relation for light
beams in the two-dimensional plane perpendicular to the direction of the beam
propagation which is more interesting for realistic beams. This uncertainty
relation connects the area of the focal spot with the spectrum of transverse
photon momenta. The shape of the beam that saturates the uncertainty relation
is very close to a Gaussian. The directions of the electric and magnetic field
vectors are those of the circularly polarized plane wave. Our uncertainty
relation for the focal spot is quite general but we were able to determine the
value of the lower bound only for beams made of many photons.
