Tyre size recommendations

I have some after market wheels on my Zed and all the tyres have perished so before it goes for its MOT at the end of the month im looking for new tyres.

The wheels are 17" and are staggered, with a 9.5j at the rear and 8j at the front.

The current tyres on the rear are 255/40/17 and a 235/40/17 on the front.

Now ive only been able to find one tyre at 235/40/17 so I want to change so I can get a decent brand, but which way do I go.

I dont want stretched look, I want a nice flat side wall look so thinking of a 245, but would this be too chunky.

The other question is if I go up a size on the front, should I also then go up on the back? Or will it not matter?

Any help would be gratefully received!