Collision Radius

Every Hero in Heroes of the Storm has a Collision Radius (also called Hitbox) which determines how much physical space their character model occupies. The Collision Radius is visible as a green (your own), blue (your allies) or red (enemies) circle beneath the model. Golden Hero Mastery Rings are a cosmetic method to flaunt your dedication to the Heroes you own. Players are able to spend to purchase augmented Hero Rings for any Hero over the Hero Level of 15 and 25. The Hero Level 15 Mastery Ring is only a circle, the Hero Level 25 Mastery Ring has "spikes" on it.

Collision Radius is used to check collision with other friendly and enemy Heroes, Structures and Terrain since those entities can not simultaneously occupy the same physical space. Heroes are however unaffected of the Collision Radius of minions, summons, mercanaries and monsters.

A target is hit by a skillshot or an AoE if the ability hits the circle beneath the model (even the edges count). Larger Heroes with a bigger Collision Radius can effectively use their bodies to block the movement of other heroes. High-level players will utilize these "body blocks" to disrupt the paths of enemies and gain a positional advantage. You can slow people down, prevent them from running behind a gate, or even pin them in an area for your team to kill them.

List of Collision Radius of Heroes
Collision Radius is used to check collision with other units (i.e. bodyblocking) and skillshots, while Inner Radius is used to check collision with structures and terrain.