An igneous rock could be of any size, from microscopic to miles in diameter.
Porphyritic Rocks are rocks that have big and small crystal's like the Igneous Rock "Andesite".
Igneous rocks with big crystals or grains form from slow cooling beneath the Earth's surface, allowing time for larger crystals to grow. In contrast, igneous rocks with small crystals or grains form from fast cooling at or near the Earth's surface, limiting the time available for crystal growth.
The small crystals are formed outside the volcano by the cold air... the cold air cools down the igneous rock and the crystals dont have time to join together so the are small. And the big crystals are formed inside the volcano very slowly and that way they have time to join together then there are big crystals that have been formed.
You would expect to find an igneous rock with small crystals near the Earth's surface where the rock cools relatively quickly, such as in volcanic eruptions or magma intrusions that do not have much time to grow large crystals. These rocks are known as volcanic or extrusive igneous rocks.
Igneous rocks can have both small and large crystals, depending on how quickly they cool. If an igneous rock cools slowly beneath the Earth's surface, it can form large crystals, while rapid cooling at the Earth's surface or in an eruption can result in small or no visible crystals.
igneous rocks
These are extrusive igneous rocks with an aphanitic texture.
Igneous rocks that result from lava solidifying above ground are classified as extrusive igneous rocks. These rocks cool quickly, leading to small mineral grains and a fine-grained texture. Examples include basalt and rhyolite.
Extrusive Igneous Rocks. Basalt (Plankey1995 JF)
Igneous rocks with large crystals are called intrusive rocks, formed from magma cooling slowly beneath the Earth's surface, allowing for large crystals to form. Igneous rocks with small crystals are called extrusive rocks, formed from lava cooling quickly on the Earth's surface, resulting in small crystals due to rapid cooling.
igneous volcanic rocks
Rhyolititcen rocks.