What would be the best tire size for 15x6 rims- to be used for spirited street driving and possible occasional auto x. I would prefer bigger tires. The body on my 914 is stock except I do have rolled fenders.

205/60-15 or 205/50-15, the higher aspect would be better for street and IMHO looks better, lower for AX.
Any bigger and you would have to at least pull your rear fenders.

195s for the stock look or 205s if you want it sticky.

It also depends on what wheels you have.
The aftermarket repros (Rivieras) will have more offset to the outside (tire touches fender lip)
The stock fuchs will have more offset to the inside (205 Hoosiers rub on the rear inside fenderwell) and might need spacers with the 205s.

However if you found a 15X6 wheel, I am guessing it is a 5 lug, and my comments above might not apply.
IF you found a 15X6 wheel in 4-lug, I would be very interested in find out where you got it.
I am desperately trying to find a set of 15X7 in. 4-lug wheels but they are the unicorns for this car.

I've got plain old 5 lug fuchs

I'd use 195 if building a /4, you're building a six so I'd go with the 205's
That's what I'm doing, mostly street so I'm using 5-lug 205/60-15.