  High-resolution simulations of star-forming massive galactic discs have shown
that clumps form with a characteristic baryonic mass in the range
$10^7-10^8~M_{\odot}$, with a small tail exceeding $10^9~M_{\odot}$ produced by
clump-clump mergers. This is in contrast with the observed kpc-size clumps with
masses up to $10^{10}~M_{\odot}$ in high-redshift star-forming galaxies. In
this paper we show that the comparison between simulated and observed
star-forming clumps is hindered by limited observational spatial resolution and
sensitivity. We post-process high-resolution hydrodynamical simulations of
clumpy discs using accurate radiative transfer to model the effect of ionizing
radiation from young stars and to compute H$\alpha$ emission maps. By comparing
the intrinsic clump size and mass distributions with those inferred from
convolving the H$\alpha$ maps with different gaussian apertures, we mimick the
typical resolution used in observations. We found that with 100 pc resolution,
mock observations can recover the intrinsic clump radii and stellar masses, in
agreement with those found by lensing observations. Instead, using a 1 kpc
resolution smears out individual clumps, resulting in their apparent merging.
This causes significant overestimations of the clump radii and, therefore,
masses derived using methods that use their observed sizes. We show that
limited sensitivity can also force observations to significantly overestimate
the clump masses. We conclude that a significant fraction of giant clumps
detected in the observations may result from artificially inflated radii and
masses, and that $\approx 100$ pc spatial resolution is required to capture
correctly the physical characteristics of star-forming clumps if they are
coherent structures produced by disc fragmentation.
