rolling diameter means almost nothing on a vehicle of this type.

if you are worried about rim size have a read of this:
http://www.imoc.co.uk/forums/viewtopic.php?t=15623

as far as picking the overall width of your tyres its a matter of preference.

a lot of people who run 15 inch wheels opt for 205 instead of 195 on the front to remove some understeer tendancies.

neither of those two options you list match up with the stock tyre widths.

if you wanted 245 on the rear you would need 210

(which may not exist?) or 215 width tyres for the exact same balance.

205 will understeer more and 225 oversteer more.

however the bigger picture is you probably wont notice any difference with whatever tyre width you pick.

dropping the front tyre pressure by 3 or 4 psi for example would probably be far more noticable for balance.
