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 Kanzelhohe 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 $2cdot 10^{23}, mathrm{Mx}$, 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 $6cdot 10^{23}, mathrm{Mx}$ 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 days 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.
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