I do not assume any explicit relationship between the link lengths, but in one of the papers (http://www.sciencedirect.com/science/article/pii/S1474667017358172 , page 119, equation no.14) I am referring assumes following relationship :
$l_1 /l_2 \geq 1.1$
$l_1 /l_2 \leq 2$
$l_2 /l_3 \geq 1.1$
$l_2 /l_3 \leq 2$
According to another paper (page 569, below equation 14), the above relationship exists in a number of the industrial manipulators they studied. What's the reason behind this relationship?