We have this Mumford's theorem: Let $X$ be a Riemann surface of genus $g$, $G$ a simple Lie group. We can consider a principal stable $G$bundles over $X$ (say $\xi$), where $rk(\xi)=r$ and $deg(\xi)=d$. For fixed $X$, $r$ , $d$, there exists a connected moduli space $M^g$ of $S$equivalence classes of rank $r$ , degree $d$ semistable bundles over $X$, which is a complex projective variety, having dimension $r^2(g  1) + 1$ when $g \ge 2$. (For reference look at this: http://www.math.columbia.edu/~thaddeus/papers/odense.pdf, page $4$). I'm interested in stable principal $G$bundles over $X$. So does exist a version of this theorem that says us what is the dimension of moduli space of stable $G$bundles over $X$?
Yes. The standard reference is : A. Ramanathan: Moduli for principal bundles over algebraic curves, I and II, Proc. Indian Acad. Sci. Math. Sci. 106 (1996) 301–328 and 421–449. The dimension of the moduli space is $(g1)\dim G$ (note that the moduli space you seem to consider is that of $GL(r,\mathbb{C})$bundles, and $GL(r,\mathbb{C})$ is not simple). 

