The main constraint at BSC is probably the Paging processing capacity : if the LAC is too big, there could be too many pagings to process for the BSC.
Don’t forget to cumulate the effect of all LACs in the BSC. For example, 4 LACs :
LAC 1: handled by BSC A
LAC 2: handled by BSC A and BSC B
LAC 3: handled by BSC A
LAC 4: handled by BSC B
The total pagings in BSC A = pagings in LAC 1 + 2 + 3
Total pagings in BSC B = LAC 2 + 4
Even if BSC A only hosts 1 cell in LAC 2, it will have to process the full amount of pagings of LAC 2.