Unbalanced data and random effects

Dear all,

I performed a census of insects at different sites and measured there size. I would like to know if size is related to an environmental factor. I modelled the size as a 
fonction of the factor with site as a random variable to account for within-site variability. However I have strong unbalanced data with some sites having only two 
individuals and others up to 100. Is having site as a random factor sufficient to deal with this strong data unbalance? The residual fit of the data is quite bad, 
certainly because of the strong difference in variance among sites. Would anybody have some advice? Thank you!
___________________________________________________________
Les pr?visions m?t?o pour aujourd'hui, demain et jusqu'? 8 jours ! Voila.fr http://meteo.voila.fr/