Can anyone tell me the dimensions of the pollen filter for this car? Euro Car Parts have a Denso filter at a good price but I'm not sure if their quoted dimensions are correct: 293x182x22mm.

Coincidentally, just had a new one delivered today so easy to measure it for you. It's 232mm x 187mm x 21mm, so the one you're looking at would appear to be wrong.

Check that you're getting an activated carbon filter and not a cheap paper one. I got a UFI (Fiat original manufacturer) activated carbon one for £7.90 delivered from this eBay seller:
http://www.ebay.co.uk/itm/Fiat-Stilo-1-9-JTD-CF3-UFI-Activated-Carbon-Cabin-Filter-02-03-/140741556726?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item20c4d9b9f6

EDIT: Just found a Denso catalogue. Their part numbers for the Stilo cabin filters are DCF039K for the activated carbon filter, or DCF084P for the paper filter.

Was the one you had delivered for a 1.4 active aircon with 95bhp engine?

No... mine is for a JTD MW 115bhp, but the cabin (pollen) filter is the same for all Stilo variants.

Here's the same filter, same price, from the same company, but listed for the 1.4 95bhp version:
http://www.ebay.co.uk/itm/Fiat-Stilo-1-4-16V-CF4-UFI-Activated-Carbon-Cabin-Filter-12-03-/140741557249?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item20c4d9bc01