The frequency being high does NOT limit the practical
length of a transmission wire, though when the wavelength
of the frequency traveling on the wire becomes
an appreciable fraction of the wire length, you will not be
able to use simple 'circuit theory' with quite so many
short-cuts of assumptions to model the flow of energy
along the wire.
It is possible for the wire to start radiating propagating
electromagnetic waves away from itself, like an antenna.
It is also possible that the wire itself in its environment
will have an high frequency complex impedance
that is relevant to consider when looking at how waves
of high frequency flow down the wire.
When the lengths of the conductors exceed around
1/10th of a wavelength of the frequency of the signal
flowing, it is a good time to start using the techniques
of transmission line design and modeling to ensure that
your wire (or transmission line) will behave as desired.
That is why there are transmission lines like coaxial cable,
twin-axial / flat two-conductor, microstrip, wire above
a ground plane, et. al. because those kinds of lines can
propagate signals over many wavelengths efficiently
without much radiation, loss, or impedance mismatch
when they're used properly.
The microwave oven has a screen of metal with holes
in it along the front door. The holes are perhaps less than
2mm in diameter, which is a huge number (4000 or so)
of wavelengths for light, but it is only 1/61st of a
wavelength at the 2.4GHz frequency that a typical
microwave operates at.
When an electromagnetic wave encounters a
uniform metal screen with performations of a diameter
less than 1/40th of a wavelength, and with good thick
metal webbing around the holes, the wave energy will
reflect off of the metal screen and only a very small
fraction of the electromagnetic field energy from the wave
will exist for any significant distance beyond the wall
of the screen.
Light, of course, passes through such a 2mm hole very
easily since the hole is 4000 wavelengths wide.