It was the first large congregational mosque of the Ottomans (1396) and measures 56 x 68 m over all.
It is covered by twenty domes. Supposedly these twenty domes were built instead of the twenty (separate) mosques which Sultan Bayezid I had promised for winning the Battle of Nicopolis in 1396.

Correspondent: J.M.Criel, Antwerpen.
Source: ‘Islamic Architecture: Ottoman Turkey’ (Godfrey Goodwin) – London 1977 - Wikipedia