By inlet pipe do you mean drain from the gbs? Outlet hose being hose from the pump?
I assume the cubes will be cut in half, so you'll have about 8 x 1/2cube beds? Pipe size depends on the amount of flow required. So if you're doing a slow drain, then small pipes may be ok. But if you want to do fast drain, then go with bigger pipes. I'd allow for both, so go with 32mm or 40mm. Bigger pipes also means less potential for blockage.
The outlet pipe, from the pump, will probably need to be bigger. But again it depends on the flow you require. Small pipes will put a lot of back pressure on the pump, and slow down your flow rate. If you plan on filling the beds slowly then use 25mm. If you plan on filling the beds fast, then use the 32mm pipe for the pump outlet, to give you maximum flow. Obviously filling fast will also require a larger pump
not sure of the price of pumps in Perth.