  Magnetic skyrmions are knot-like quasiparticles. They are candidates for
non-volatile data storage in which information is moved between fixed read and
write terminals. Read-out operation of skyrmion-based spintronic devices will
rely upon electrical detection of a single magnetic skyrmion within a
nanostructure. Here, we present Pt/Co/Ir nanodiscs which support skyrmions at
room temperature. We measured the Hall resistivity whilst simultaneously
imaging the spin texture using magnetic scanning transmission x-ray microscopy
(STXM). The Hall resistivity is correlated to both the presence and size of the
skyrmion. The size-dependent part matches the expected anomalous Hall signal
when averaging the magnetisation over the entire disc. We observed a
resistivity contribution which only depends on the number and sign of
skyrmion-like objects present in the disc. Each skyrmion gives rise to 22$\pm$2
n{\Omega} cm irrespective of its size. This contribution needs to be considered
in all-electrical detection schemes applied to skyrmion-based devices.
