Hi welcome to the fiat forum.
Its proubly not in the hand book because fiat want you to go to them and buy theres. Dont know if mk2 is same as mk1 but mk1 has two difrent size ones cant remember off hand what these were, but i measured them with a tape mesure then bought 2 packets of £1 blades off the market that were as near to the size as i could get, they work great if you clean them with washing up liquid before use.
I think mine had 21" and 17", but Halfords booklet thingy said 21" and 18". Plus they didn't have 17". The 18" fits fine. A tip for you all: I went for the expensive Bosch ones with spoiler etc, rather than cheap ones like Fiat fit. It's worth every penny IME - they are silent and clean so well - okay my others were 3 years old and dreadful, but it made me happy to drive in the wet again!