Steve
I wouldn't disagree with what my learned friends have advised but if you want to "fill the arches" the offset is just as important.
If you do it right the rolling radius stays the same so in profile the wheel/tyre combo will be no bigger. However, what gives it the aggressive look is how far the wheel is tucked into the arch when you look at it side on.
For example, a high offset (eg +42) on a Zed will leave the wheels tucked deep into the arch and looking like a canoe (think E-type Jag). On the other hand I've got +18 on my 9" wide fronts and that pushes the wheel right out to the very edge of the bodywork - very aggressive looking! Any more than that and they stick out of the arches which is a look best reserved for chavved Corsas.
I can't just tell you what offset to go for as it depends on the style of wheel and width of wheel. I'd suggest that if you decide on a wheel you like and post it up we should be able to give you some more of an idea what sizes, offsets, tyres etc would suit.