It can be a.) mechanical or b.) electrical.
 which one are you interested in and what frequency range?

<nbucska@pcperipherals.com>

It doesn't matter if it is mechanical or electric resonator. The main conditions are that the size ( for example lenght ) has to be half of the wavelenght on desired frequency or multiple of this lenght and the input of the resonator has to be matched to the source.

A pocket radio containing a resonator tuned to 500m
( cca. 1500 ft.) is somewhat smaller than the wavelenght.

The impedance has to be matched to the source if you want max. power. Very often there are other, more important considerations e.g. selectivity.

<nbucska@pcperipherals.com>

In electric devices You can change the size of resonator by using lumped components. For example in wire antennas if You add inductor, the electricel lenght will be bigger. According to it You can make antenna shorter physically but the electrical lenght is the half of wavelenght.
Also it depends of the material Your are using to build resonator. For example in dielectric resonators, wavelenght in dielectric material is shorter then in free space. So, You can achive smaller dimensions of resonator. It depends on dielectric permability. There is a lot of possiblities.
       The conditions I've mentioned are generall. There are of course lot of other ways to achive resonans.