Using the traditional instruments for measuring high frequency signal
Not sure to understand what you are asking, Microwaves wavelengths range between 300 MHz (1 m) and 300 GHz (1 mm) which is indeed of the same order of magnitude as the probe. 

1GHz corresponds to a wavelengths of about 30 cm  and is also in the Microwave range. 

Are you saying that it is possible to measure a frequency of 1GHz with a probe while it is usually said that Microwaves can't be measured with it?