Engine rebuilding

To measure cranks and pistons what size micrometer should I purchase

What are the rough sizes of the pistons and journals? You need a micrometer that can measure that range (they usually measure about 1" / 25mm). So for a 84mm piston, you would need a 3-4" (75-100mm) micrometer. For a 1.2" journal, you would need a 1-2" (25-50mm) micrometer.

Typically I'd suggest you purchase a set of micrometers. At a minimum you'll at least need a couple - One that would suit your crank journal diameters, and another to measure your piston diameter. By the time you buy two, you'll likely find that a decent set of Chinese micrometers will be a similar price. The set that I use in our webinars and courses was sourced from Jegs and I think was about $300-350 USD

You will also need internal mic's for the bores, or at least snap guages.