In this work, we bridge fundamental electromagnetics and chemical process engineering with the aim to develop tailor-made (microwave or high frequency radiowave) applicators for heating of micro- and small-structured process equipment. In this context, two simple configurations with well-defined single mode field patterns, namely a cylindrical and a rectangular cavity both containing a homogeneous cylindrical load were analyzed either analytically or numerically. We present design charts that illustrate how important operating, geometric and materials parameters relate with each other. It was found that load size, heating uniformity and desired frequency mutually constrain one another. The required cavity volume increases with increasing heating uniformity or with increasing load permittivity for a given heating uniformity requirement. At the popular frequency of 2.45 GHz the load is restricted to a small size, compared to the cavity size, in order to achieve high heating uniformity. Opting for lower resonance frequencies allows for bigger load volumes to be heated uniformly. Furthermore, we show that the relations found for the operating, structural and material properties on the basis of these simple configurations can provide design guidelines and first approximations for more realistic process equipment geometries.