Syncro how big can I go

The general consensus of the biggest wheel you can safely fit on a Syncro 14 is a 215/75/15. This is slightly bigger not only in tread width but in overall diameter so your cruising speed will be slightly better too.

The absolute maximum will be defined by what sort of tyre you are running, as they can vary considerably in circumferance from manufacturer to manufacturer. It's certainly possible to run 225/75 R15 AT treads, but more aggressive treads might foul at this size