correct sizes of boxes individual packing to ship to amazon

Hello everyone.

i am trying to send 3 boxes of my product to amazon (i already am selling with private label on my own listing). I packed these products (single SKU) into 3 boxes. The longest side for the box is 75 cm. When I try to create shipment on seller central it says that the longest side needs to be 63 cm or shorter.

Would anyone know where can i find information on correct sizes of boxes or weights that are allowed to pack as individual packs (not a pallette) and ship to amazon?

A bit confused on what to do next.

Many thanks.

Aya