The von Bertalanffy growth function can be used in evolutionary models (Roff 2010, page 132). It is also apparently a standard growth function used in modeling fish growth.
It can be expressed as a differential equation as
L'(t) = r(B)*(L(inf) - L(t))
where r(B) is the growth rate
L is length
and L(inf) is the ultimate size of the organism.