Unless you really,really need it to be a single PCB then it will usually be cheaper to split into smaller boards, as the choice of manufacturers reduces significantly above about 500-550mm long.
And of course if you want to get it assembled, you have the same issue of reduced choice of subcontractor.
For large boards, shipping costs can also be disproportionally high, both due to the size, and the extra weight of packing needed to protect the boards from shipping damage.