Well its more a case of what you like the look of to be totally honest mate, as a Back Box on its own isnt going to add hardly anything in the way of performance.
Its generally excepted that anythink around the 3" to 4" max looks most at home on the Saxo, but some people are leaning towards more motorsport inspired exhaust with very small tail pipes around the 2"
Personally I like Pipers on Saxos as they do a very nice Slash cut tailpipe that doest have a inwardly rolled tailpipe like the Magnex, Janspeed and so (basically a funnel inside the tailpipe).
Heres a collection of Magnex's on saxos - I feel the Can hangs too low and is far to big - hence you can see half the rear siliencer rather than just the tailpipe.
Ask people to post pics of what there running as see what takes your fancy.
Dual Exit Scorpion looks very tasty if thats your thing :-)
