  We review the effects of source size in interferometric observations and
focus on the cases of very compact sources. If a source is extremely compact
and/or weak (so it is not possible to detect signature of source structure in
the visibilities) we describe a test of hypothesis that can be used to set a
strong upper limit to the size of the source. We also estimate the minimum
possible size of a source whose structure can still be detected by an
interferometer (i.e., the maximum theoretical over-resolution power of an
interferometer), which depends on the overall observing time, the compactness
in the array distribution, and the sensitivity of the receivers. As a result,
and depending on the observing frequency, the over-resolution power of
forthcoming ultra-sensitive arrays, like the Square Kilometer Array (SKA), may
allow us to study details of sources at angular scales down to a few micro-as.
