To determine fin size, go measure the tail of your board 30cm from the tip. A heavy sailor would be able to cope with this width plus 2 or 4 cm, a light sailor would require at most the width itself. So as a rule of thumb, if the 1' off is 52cm you can then use up to about 54 or 56 max.
This width is commonly known as 1' off ie the width of the tail 30cm/12" from the rear. For more details on why this matters read
http://bluefinz.com/html/Technique/Fin%20selection/fin%20selection.htmThe max fin size does to some extent determine max sail size. A silly crude ratio I have developed over the years testing a wide range of boards is..
reaching : max sail size is fin size / 5
upwind: max sail size is fin size / 5.8
dont take this too literally, its just a crude formula but it works reasonably well..
so..say a 36cm fin..the sail size for reaching would be a 7.3 (pretty much spot on, I use either 7.0 or 7.6)
...say a 52 cm fin...for upwind.. the sail size would be a 9.0, for a 66cm fin it would be 11.3 ..
there can be variation from this but when in doubt i find this a good guideline...again...just a guideline
so..lets take your 9.8, for reaching you can get away with say a 49 cm or thereabouts. for upwind you will need at least a 56. I don't have specs of the titan but i'd guess the tail width to be about 52 cm..in which case you can use 2 fins say..a 56 as your biggest (or 54) and a 48 as you "high wind" fin.
last point: at your weight a 9.8 is a small sail. We have a gentleman using an 11m nitro IV at your weight and even then he needs about close to 10knots to get planing on a GO 170liter.
heck..hope that loonnng answer makes sense..if it doesnt give me a call..