Image below is for a 9j wheel with a 225/40 tyre ET35. the orange square is the size of the new wheel and the black lines coming from the corners is how the tyre will fit.
Tyre Size CalculatorThe green square is the original wheel with the original size tyre

Now 245 tyres

Now 255

etc
Now for the fronts
215

225

235

Other things to remember is if you are low on the rear a wide tyre may rub the arch so the stretched tyre will give clearance. And on the fronts the tyre may rub on the strut or coilover adjusters.
Also bigger tyres are more expensive so are the non popular sizes.
Heres a pic of my 9J ET30 wheels with a 225/40/18 tyre
