  We study the energy release process of a set of 51 flares (32 confined, 19
eruptive) ranging from GOES class B3 to X17. We use H$\alpha$ filtergrams from
Kanzelh\"ohe Observatory together with SDO HMI and SOHO MDI magnetograms to
derive magnetic reconnection fluxes and rates. The flare reconnection flux is
strongly correlated with the peak of the GOES 1-8 \AA\ soft X-ray flux (c=0.92,
in log-log space), both for confined and eruptive flares. Confined flares of a
certain GOES class exhibit smaller ribbon areas but larger magnetic flux
densities in the flare ribbons (by a factor of 2). In the largest events, up to
$\approx$50\%\ of the magnetic flux of the active region (AR) causing the flare
is involved in the flare magnetic reconnection. These findings allow us to
extrapolate toward the largest solar flares possible. A complex solar AR
hosting a magnetic flux of $2\cdot 10^{23}\, \mathrm{Mx}$, which is in line
with the largest AR fluxes directly measured, is capable of producing an X80
flare, which corresponds to a bolometric energy of about $7 \cdot 10^{32}$
ergs. Using a magnetic flux estimate of $6\cdot 10^{23}\, \mathrm{Mx}$ for the
largest solar AR observed, we find that flares of GOES class $\approx$X500
could be produced ($E_{\rm bol} \approx 3 \cdot 10^{33}$ ergs). These estimates
suggest that the present day's Sun is capable of producing flares and related
space weather events that may be more than an order of magnitude stronger than
have been observed to date.
