what kite is faster-12 or 10m in same conditions

bigger kite = faster

Now I'm no race expert but i think if its either 10 or 12 , you don't want to be ridiculously overpowered, but you want some juice! So in identical conditions probably 12 would be faster but in 22 knots as a 67kg rider you might be better lit on the 10. What boards?
On a surfboard I'd def go 10.

Salt i have never raced so i am only talking from a season and a bit of kiting but i would stay with what you know and are comfortable with.

As you say it's too easy to crash out & blow the race.

I reckon you should stick with your own kite & board unless you have the luxury of time to train with the new gear.

With your weight you should be stonking along on a 2009 10.5 helix; have a little faith in yourself

A bigger kite will generally 'fly' slower. However a bigger kite will generally also have more power and thus may pull you faster in the same apparent wind. Though the aspect ratio ( and lift drag ratio) and projected area details (eg c vs bow and angle of attack) will also influence the 'speed' of a kite and it's performance both up and down wind. A 12m kite will have more power than the same model of 10m kite and will therefore enable you to move faster as you power through the chop, but the 10m kite will turn and fly across the wind window faster, with it's own results.

Bigger kite = slower not faster

this is assuming there is enough wind for the smaller kite and that the rider has decent technique.

A smaller kite has less drag and will want to fly further into the wind window also allowing greater upwind ability or speed.

A bigger kite has a lot more grunt/ torque due to a larger leading edge and canopy etc.... Creating more drag the kite will then have a tendency to sit further back in the wind window.

The faster the kite can fly the faster you can go !!

If it is a downwind race the physics of drag and the kite is negated . But you may still want to use the smaller kite as you can loop or figure 8 it to gain downwind speed, by consistently looping the kite whilst riding ( not jumping ) you can take a more direct downwind approach ie: go straight down wind. This I harder than it looks and you need to be wary of nosediving your board and keeping tension in your lines, you can keep the tension in your lines by making small carves as you go down wind.

This will see you take a more direct line to the finish whilst others tack in and out bearing off the wind covering 3 times as much ground as you are and therfore you will be way ahead, if it's a serious downwind race and you want to win use this method .......if it's a fun social downwinders/race stick with the mob and enjoy the jumping etc... As you make your way to the finish.

I wouldn't worry about extension lines in this scenario as they tend to slow the kites response to turning inputs if you are taking the straight downwind approach you want your kite to respond quickly to the turns loops and eights

line extensions are really only good for increasing your power stroke ( creates a bigger arc) which is good for light wind or for areas where the wind can be a bit higher up. Ie: 10 knots at sea level , 15 knots about 10 m up.

^^^^^yep,,I think that everytime I sober up,,thats not very often..hik-up

bigger kite = faster RIDER
smaller kite = faster KITE

enough of the tech talk....

True a smaller kite is faster through the air and can build apparent wind but if you want to get around a race course choose the right kite for the conditions.

A 67 kg rider in 22 knots on a 10m kite will have better control, better up wind ability hence be faster round a course than the same rider on a 12m riding at the edge of his ability. Also, some kites have better upwind ability than others.

Don't forget the board too. For speed you need a flat board that can edge up wind as well.

Recent races on Botany Bay saw an experienced rider on an 8m beat experienced riders on 12m kites more than once.

It all comes down to rider/kite/board/luck