From Quake Wiki

A unit is Quake's internal measurement of size and distance. It is not mapped to any real-world measurement, so the generic term 'unit' is used. When making levels for the game, units are often counted as multiples of 16, typically 16, 32, 64 and 128 for normal scale brushwork. It is helpful to think in these sizes, because the textures used in quake are usually the same dimensions, and 1 pixel of a texture mapped at 1:1 scale is equal to 1 unit in space.

The size of the player in units is a box with dimensions 32x32x48. The player's default movement speed (as defined by sv_maxspeed) when running is 320 units per second, and for most purposes, it is safe to assume the maximum jump height to be 32 units.

While there is no official comparison to real world measurements it's generally assumed that 64 units equals approximately 1.7 meters.