For Toyota Previa owners in the UK, Tyres.co.uk lists the factory fitment for the second generation of the model as Continental UltraContact and Bridgestone Turanza T005 at 205/65 R15 94H. If you confirm the load and speed ratings on the placard and the vehicle registration, these can be fitted to the car.
To find the tyre size of a Toyota Previa, look for the driver's side inner B pillar. There is a tyre information placard that will have the size and the load and speed index, as well as recommended cold inflation pressure. Once that is confirmed, you still need to check that the size actually fitted is the same on the tyre sidewall.
The UK version of the second generation Toyota Previa is a front wheel drive, so you only need to replace one axle at a time. However, if your Previa is a four wheel drive version, you should maintain matching pairs on all four positions. If you replace just two tyres, put the new pair on the rear axle and the rear tyres on the front axle.