You can hold me in your hand and yet I can fill the entire room. What am I?

“A balloon”

While a balloon can fill a room with air when inflated, it cannot be held in a way that allows it to light up the space.

“A smartphone”

A smartphone can indeed be held in your hand, but it doesn't fill a room in the sense of illumination like a light bulb does.

“A speaker”

Although a speaker can fill a room with sound, it is not typically described as something you hold in your hand and is not related to the concept of light.