1. Can't remember exactly but 7L vs 14L springs to mind. The 748R airbox is huge by comparison to anything else bar a factory Corse one.
2. Yes, but it's a fair bit of work. You either have to cut the airbox to fit over the "X" section of the frame between the cylinders and then fabricate some sort of tunnel that'll seal the airbox again, or, chop out the 998 "X" section of the frame and replace with 748R style "X" section.
3. They appear on eBay very occasionally but remember that the 748R airbox is in 2 pieces and generally it's only the top half that comes up for sale. A carbon upper airbox from Ducati is circa £400 and you'll also need the bottom half, not sure of price but not cheap!
4. Yes, you'll need to make up some brackets to secure the lower part of the airbox to the engine.
5. Not done it to a 998, but the principle is the same.
You're best bet would be to source either a genuine 998 Corse airbox (you'd need to do other mods to fit it though) or buy one of
these.
Hope that helps!
Neil.