Most of the ones I've seen are 80mm (about 3 3/4"). That's a standard gauge size. Unfortunately, high rpm tachs that small are going to be rare as the demand isn't there. Custom bikes will usually come nowhere near 10k rpm's, nor will most race cars. Karts will, and you can find more there, but again, you're going to find most are 80mm.
Wiring should be easy. Depends on the gauge. May wire into the existing tach wire. May sit on the spark plug wire and watch for spark.
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