Determine the size of css classes inside css using twitter bootstrap

I am migrating my small business site to bootstrap to be mobile. I'm still familiar with the bootstrap base and I'm not a css expert either ...

My question is conceptual. Today with my old site using html and css, I define in my CSS both the size (yes, fixed sizes) and styles (color, etc.) of the Sections I am using.

On the other hand, in all the examples I've seen in bootstrap, it is possible to style the div only in css, and the size is determined in html when the bootstrap column is sized (e.g. col- cm-4). [Note. I follow to have two css, bootstrap and my custom one.]

The way I see it, I am losing the ability to size divs centrally from css and therefore lose maintainability. Also, making the site will be more difficult as the col-sm- * classes are semantically poor. It's right? Did I miss something?

Is there a way to define css classes that reference bootstrap classes? for example Define a .menu css class that can behave like .col-sm-4.

Any suggestion or recommendation is greatly appreciated.

Thank you in advance