I'm not sure if this is a question about preprocessor syntax or implementation detail (or maybe there's an unseen design consideration). The sample code you put up won't compile, so are you confused as to syntax?
There is a log function in math.h and you could use that (and maybe put it in a function rather than a macro).
There are various ways to figure out the position of the highest bit for integer values as well, which may suit your needs (see
http://graphics.stanford.edu/~seander/bithacks.html)
You asked for a very specific implementation detail with no surrounding design context. How are you using SIZE_WIDTH (what are you using it to do)? I mean, whats the overall task you're trying to accomplish? Maybe there's a better approach to be taken.