I searched the Internet for largest known galaxy, and its IC 1101.
Wikipedia puts its diameter at 5-6 million lightyears.
In Celestia, its radius is 1.832e+05 ly, which would only be a diameter of 366,400 ly.
It seems that there's a little discrepancy here.
So which is wrong?
The linear diameter in ly of a galaxy is measured by means of its angular size in radian and it's distance in ly:
angular size =diameter/distance
Both the reliable RC3 galaxy catalog and the SIMBAD world database give a similar value for the angular size ~ 1.2 - 1.4 arcmins. However, the distance of IC 1101 is largely unknown! The paper that is referred to in the IC 1101 Wikipedia article (that you presumably used) is not a standard accepted scientific reference. Correspondingly, there is no distance value of IC1101 in the NED1d catalog on galaxy distances. As a rough estimate of the distance I therefore used Hubble's law for Celestia's data base (galaxies.dsc). The resulting diameter and distance are consistent with the observed angular size.
If you want a much larger diameter, the distance must be also much larger, in order to match the reasonably well known angular size. In any case there are very large uncertainties involved. Since IC 1101 is the central object in a galaxy cluster, the distance from Hubble's law (and the spectroscopically determined redshift z) might receive large corrections due to so-called peculiar (i.e. non-radial) velocity components
induced by other gravitating cluster members!