Order of parabolic subgroups of affine Weyl groups

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I have a question about computing the order of an arbitrary parabolic subgroup of an affine Weyl group $W_a$. Given a proper subset $I \subset S_a$ associated with the reflections for the fundamental alcove $A_0$, is there a formula for the order of the parabolic subgroup $W_I$ generated by $I$? How can one prove it has to be finite?