Correct order of S-S bond length in the following oxoanions is 

i)  $S_2{O}_4^{2-}$     ii) $S_2{O}_5^{2-}$     iii) $S_2{O}_6^{2-}$

With increase in oxidation state, the size of atom decreases. In $S_2{O}_4^{2-},$ both $S$ are in $+3,$ in  $S_2{O}_5^{2-}$ one   is in $+3$ and another S is in $+5$ in $S_2{O}_6^{2-}$ both $S$ are in $+5$ oxidation states.