1)
Why did he make so many lavers?
1.
Rashi: "Es Ma'ase ha'Olah Yadichu Vam" (Divrei ha'Yamim II, 4:6; Rashi there - their water is used to rinse innards 1 of Olos. However, the owners eat Shelamim in the entire city.) Radak - they are also to wash the hands and feet of Kohanim. Radak there - the prefix Vav is omitted; it means v'Es Ma'ase ha'Olah
1
The innards of almost every Korban are burned on the Mizbe'ach! Regarding the limbs, Shelamim is unlike Olah! (PF)
2)
What were the dimensions of the lavers?
1.
Refer to 7:27:2:1.