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I am developing an architecture application with webgl where i render some 3D shapes and i need to display the dimensions.
To express myself better i ask you to refer to this screenshot (found on google):

Somehow in unity i already did the "3d shape work" and now i need to "draw" over it the lines with the dimensions.
I do not want to use small arrows (like in the picture above) but just simple lines and writing numbers close to them.
When i zoom (=when i move the camera) i do not want that the lines become bigger.

I tried with tiny cilynders but on zooming the cilynders stop to be lines and become "bold objcets".

I think i am missing some basic technique here. I hope you can suggest a way to perform this task. Thanks.
Thanks, Vectrosity seems to do what I need, I do not understand why a so basic thing is not part of Unity itself, but the price is not high at all.
About numbers i am ok with unity 3d text.

Thanks.
Because it's not a required/needed tool used by very many developers to create games, unlike the sprite packer or mecanim, or animator, particle systems etc etc.
Did you take a look at the LineRenderer? We have also made some improvements to it in 5.5.