Jamieson, Fausset and Brown on Jeremiah 52:21

21. eighteen cubits--but in 2Ch 3:15, it is "thirty-five cubits." The discrepancy is thus removed. Each pillar was eighteen common cubits. The two together, deducting the base, were thirty-five, as stated in 2Ch 3:15 [Grotius]. Other ways (for example, by reference to the difference between the common and the sacred cubit) are proposed: though we are not able positively to decide now which is the true way, at least those proposed do show that the discrepancies are not irreconcilable.