I would guess you are going to use the 12m alot at your size because it should get windier before too long (let's hope). With use of a larger board you should get up and go in about 13 knots and with time, feel comfy up to about 22 or so.

A 9m would overlap that a tad at around 18 knots at your size and keep you going up to about 30.
The first season is the BEST! So much fun and always improving.
Prepare for addiction.

Until you experience 20+, its hard to relate, but I recommend being extra safe in the 25+ stuff. Especially the first time.
Its a TON of power and stuff happens fast.