The main contenders for largest-ever bird (all extinct) are the South Island giant moa (Dinornis robustus), the elephant bird of Madagascar (Aepyornis maximus), the giant mihirung (Dromornis stirtoni), and the giant phorusrhacid Brontornis burmeisteri. Body masses of between 300 and 500 kg have been suggested for all of these, using a variety of limb bone measurements and sometimes dubious scaling models based on living birds. I found a combination of limb bone measurements (femur and tibiotarsus diameter) improved the accuracy of mass estimates, but the margin of error is still huge when extrapolating much beyond the 100 kg ostrich.
Pachyornis elephantopus skeleton, sent to Richard Owen at the British Museum by Walter Mantell (son of Gideon Mantell, the Iguanodon discoverer). P. elephantopus weighed over 200 kg. Yes, wingless terrestrial birds do retain a rear toe.