Question

How grain size of the igneous rocks varies from fine to coarse?

Answer 1

The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling ... or grain size: intrusive rocks are course grained (all crystals are visible to the naked eye) while extrusive rocks may be fine-grained (microscopic crystals) .

Answer 2

Fine grained rocks are called "extrusive" and are generally produced through volcanic eruptions.

Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic.