By default, Blender units don't make any attempt to relate to real-life measurements. However, after exporting a mesh, the slicing software will interpret the Blender units as millimeters, generally. So, it is good to think of Blender units as millimeters. This means that default objects in Blender are 2 mm across when they're added, which is fairly small.

Remember that one blender unit will be 1 mm when the file is used to 3D print an object.