Hi Adz- the standard ST24 exhaust is 2.25" diameter, so your's smaller than standard,that certainly wouldn't help your power output....
Don't go too large though, I noticed someone on fordmondeo.org suggested 3" diameter
. Have a look at what size other people on here are using- 2.5" would be plenty big enough, any larger and you run the risk of slowing exhaust velocity and making things worse. Personally I'm sticking with 2.25" and getting low restriction mufflers to replace the factory ones.
