What does "physical dimensions approaches the wavelength" mean exactly?
If the frequency is (say) 2GHz, then the wavelength is 150mm.

If the physical dimensions are big enough that they are said to "approach" (might be 7mm or 15mm) then you need to employ EM theory. 

If the part is (say) an 0603 resistor you can treat it as a lumped element at those frequencies. 



This use of "approach" is kind of a quirk in English. It just means that if you imagine the parts getting bigger and bigger at a given frequency so the sizes "approach" the wavelength then the parts no longer behave in such an ideal manner.