Depends on the engine. The Mk 3 fiesta was fitted with a variety of petrol engines and a diesel option. Petrol variants often take a bit less than 4 litres of oil, in which case a 4litre pack will give you a bit left over for topping up, whilst the diesel engines of those days often required more than 4 litres, so a 5L pack will leave a bit for topping up. If you are not sure which engine you've got, buy a 5L pack.
H4
just about everything
13.5 cm
My Mk3 fiesta did not.
2.5 liter
The rear speakers in the mk3 are 5 1/4 inch. If looking to install different speakers, an adaptor may be necessary.
Pretty close to 5 quarts.
I am not aware of a 2002 Ford Fiesta! In the US?
usually its 5W30 but this may differ depending on the engine, only really if its an RS model or XR2. though most motor shops (halfords definately) have a guide book in their store with details of which oil to use.
easy two plastic screws under seat
http://www.fiestaguides.co.uk/content/interior.html
No, the floor pans are completely different. The MK3.5 will fit the MK4.