The Winter Palace in St Petersburg is the largest of all. Built between the late 1730s and 1837, it has 1,786 doors, 1,945 windows, 1,500 rooms and 117 staircases. Its main façade is 150 m long and 30 m high.
The Royal Palace of Stockholm is the largest still in new for its original purpose. Erected between 1697 and 1760, it is also one of the oldest Classical palaces after Versailles (its construction started 13 years before the completion of Versailles). This large block of 230 m on 125 m contains 1,430 rooms.
The Royal Palace of Brussels has a longer façade than Buckingham Palace. Constructed as the Classical palace from 1775 on the ruins of the Coudenberg Palace, the former seat of the Dukes of Brabant, the palace became royal in 1831 and was modified and enlarged by Leopold II between 1862 and 1865. There is unfortunately very little information available regarding its size. The even larger Royal Castle of Laeken, where the Belgian Royal family now live, is all the more mysterious (and closed to the public). Judging from Google maps it forms a U with a core roughly 300 m in length with wings of 200 m. Unfolded it would create a 700 m long façade, against 500 m for Versailles.
Herrenchiemsee Palace, 60 km south east of Munich, is a Bavarian copy of Versailles. Built between 1878 to 1886, it reproduces the main features of its French counterpart, including the Hall of Mirrors and the Ambassador Staircase (now lost in Versailles itself). Its construction cost 125 years ago was estimated around 100 million euro at current value.
Schönbrunn Palace in Vienna is contemporary to Versailles (it was built between 1696 and 1701), but represents the Baroque style, the opposite current of Neoclassicism. It has 1441 rooms.