I've sailed the GO 170 with a 70 cm fin, and it works pretty well.
Depending on your weight and the sail size, 70 cm may be a little too much.
I weighed about 160 lbs. (72.5 Kg.) and for me a 66 cm actuall worked better.
You must also consider changing fins to maintain control and get better top speed.
A 70 cm formula type race fin will fit in the GO 170 nicely, and you will get the absolute best early planing (for whatever sail size you choose) with the 70 cm in < 10 knots.
At around 12 knots..maybe 14 knots, with the narrower tail on the Go 170, you need to think about changing to a smaller fin.
Too much fin will ultimately result in "tailwalking" at some speed. Once you find that speed (based on your experience on your board; with your rig and fin) you will know when you need to change down to a smaller fin and probably a smaller rig.
If you have the water depth, a big 70 cm fin is key to getting planing as early as possible.
Also a grunty free race type sail with lots of draft (or that can be rigged with lots of draft) is the other key.
Hope this helps,