Need more information about what is there now. If the existing subfloor is planks, then the minimum thing you need to add is 1/2" C-faced or better plywood with exposure 1 or exterior rated plywood - no D face. If it is plywood, then the minimum thickness you should consider is 3/8". You probably can't find thinner stuff that meets the specs to install tile on anyway.
But, all of this is mute if you don't have enough strength in your joist structure. To get this, you need to know the species of the wood, its condition, how thick they are, how tall they are, and their spacing. Then, the deflection can be determined. The size of the room is irrelevant, it is the structure under the floor that is.
CBU works, but I really like Ditra from www.schluter.com. It is lighter, easier to install and cut, and provides more insurance for the floor. Especially on a small room, it is far easier, and the cost isn't a big delta.
For the plywood to actually help, it needs to be in big pieces.