  We investigate, using numerical simulations, the conformations of isolated
active ring polymers. We find that the their behaviour depends crucially on
their size: short rings ($N \lesssim$ 100) are swelled whereas longer rings ($N
\gtrsim$ 200) collapse, at sufficiently high activity. By investigating the
non-equilibrium process leading to the steady state, we find a universal route
driving both outcomes; we highlight the central role of steric interactions, at
variance with linear chains, and of topology conservation. We further show that
the collapsed rings are arrested by looking at different observables, all
underlining the presence of an extremely long time scales at the steady state,
associated with the internal dynamics of the collapsed section. Finally, we
found that is some circumstances the collapsed state spins about its axis.
