There isn't a way to do this in Game Guru itself, you will need to open each model individually in a 3D modelling program like Blender to both find out what the polycount is, and potentially reduce it. Some 3D modelling programs have tools to reduce polycount; Blender has modifiers that do this (decimate I think is one), Milkshape has a simple DirectX mesh tool with a simple slider you can pull down to lower your polycount, and there are also dedicated tools out there like MeshLab that have multiple tools to reduce polycount.
Yes, the filesize does relate to the polycount, but so too does animation. Models with animation are a little bit larger than models without.
Overall though, it should be noted that meshes themselves are relatively cheap memory-wise compared to textures. You're way more likely to run out of VRAM with your textures than with your models.
EDIT: For reference, a single 512 x 512 texture (with no alpha channel) takes up as much memory as a 16K poly mesh.
AE