OEM ferrules are pressed on by machine. They are not designed to be removed.
Ferrules are cheap tho- cut the housing to the length you want,then re-cap them with new ferrules. The LBS should have them. Bike shops buy them in 100's or 500's.
There are 2 different sizes: 4mm and 5mm. 4mm is usually found on modern higher end road and mountain bikes. 5mm is usually speced on mid to low end bikes. The diameters are not interchangeable.
There are also 2 different types of ferrules- Brake and derailleur. The brake ferrules are pressed with a larger diameter hole on the end. The derailleur ferrule is machined and thicker at the end to prevent SIS housing form pulling through. At home, I normally use 5mm derailleur ferrules for everything as it's easier to just keep one around so i don't make a mistake, and the price difference is minimal in the grander scheme of things. Do not use brake ferrules on SIS derailleur housing- the housing pulls through.