How can I change the font size of a fraction in an index?

I have, as part of an equation, the object

a^{5} \left(\frac{b}{c}\right)^{\frac{2}{3}}

The problem is that the 2 and 3 are very small (compared to the 5) and I would like all sub/superscripts to be the same size just as all body text is the same size. Is there a way of defining the font size inside the equation environment?

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Have you tried dfrac in place of frac? –  56phil Sep 5 '12 at 16:42
I suggest writing ^{2/3} or stick to \frac. –  egreg Sep 5 '12 at 17:36