Is there a default mass of a func_physbox? Is it depending on its size? If yes, how big is this mass/V?

Generally it works it out by how much volume of brushes there is and what preset type of object it is.
Making a medium to large cube and then turning it into a physbox will generally be too heavy to lift, but a hollowed cube will not.

There is also a mass scaler inside the physbox properties where you can reduce or increase the mass depending on what you want to do. Usually the physbox is too heavy therefore put a lower number in order to make it be able to pick up. E.g. "0.1" (no quotes otherwise you'll break hammer).
A number higher than one will increase the mass.

Not entirely sure on how much mass/V it is though.

To find out what current mass your physic objects have, type "developer 1" or 2 in the console. After that, aim to your physical object and enter "ent_text".

thanks, those infos were exactly what I needed.