Hi, first of all thanks for your suggestions !
In the current version it's possible to modify the scale of each model, turning off Geometry->Use Objects Dimensions and modifying Width/Height values.
Anyway, since this procedure was designed initially for billboards, we have modified it in Forest 4.3, adding a local Scale value specific for Custom Objects.
We have plans to release the first beta of v.4.3 in the next weeks.