Higher the temperature more disorder you have in your system. 

So in higher temperature the crystal should have smaller size than in lower temperature, because of the perturbation of the system. 

you should also consider another situation that is if the temperature is decreased rapidly the solubility of your product will be significantly decreased in a short time (generally), thus the crystallization procedure will be accelerated leading smaller crystals. 

Each phase is stable only in a range of P(ressure), T(emperature) and C(omposition). Besides, the velocity of changing these parameters affects the kinetic of crystal growth.

Dear David,

                               during   your  hydrothermal synthesis, there is a point (at given pressure P and temperature T) where your solution is at equilibrium. This means that you cannot crystallize nor dissolve your zeolite.  Let' s Peq and Teq  be the equilibrium conditions.  Let's assume that the solubility C of your zeolite increases with T. This means that if you want to crystallize, you must impose a crystallization temperature Tcr < Teq.

In this case your system becomes supersaturated and the supersaturation is  S= (Ceq-Ccr) /Ceq.  Then S increases when Ccr decreases. From the rules of the crystal nucleation it follows that the number of formed crystal nuclei  increases with the supersaturation while their size decreases with S.  This is the reason why when  the crystallization temperature changes, the crystal size changes as well...

Yours sincerely

Dino