I changed mine a while back, I can't remember 100%, but pretty sure the standards are 13cm.
I upgraded mine to 16.5cm Pioneers (pretty sure), and I didn't bother buying adapters, they're held in with two screws tightly (no mods required) and they don't rattle and have been in there for 5/6 months.
I don't really want to spend too much money on the van so saved myself money on buying adapters which I could get away without using.