You create a default cube. It is 2/2/2 BU in size.
If you got no units set, that’s your indicator. Once you zoom in, the grid subdivides further into 1/10th of 1 BU and so on.
It’s your job to keep track of it, and if you look in the properties panel of your objects you see the sizes.
If you set Blender to work in (I guess you’re from Finland, so) meters, it’ll show you the current size of one grid segment in the upper left.
kilometers, meter, 10 centimeter, centimeters, millimeters…
In the objects properties you can even use micrometers.
If you have a cube that is 2m wide, you enter edit mode and look at the vertices, the vertices are 2m apart.
If you scale it down now by 50% the edges are 1m long, back to object mode, the cube is now 1m wide.
If you scale the cube now down by 50% again in object mode, the DIMENSION is half a meter alright, but the object scale is no 50%. If you enter edit mode again, the vertices are still 1m apart and not 50cm.
Back on object mode, if you apply scale (ctrl+a) the scale snaps to 1 again, the object dimension is still 50cm, but if you go to edit mode now, your edges are also 50cm long.