We have not seen your model, but if it was not created for games, it might contain way to many polygons.
If its more than just a few thousands, than you should try to find another model instead of trying to get this model small enought to fit to the engine.

Some models are made for rendering in animation films.
If you open a model in MED, you can open menu FILE/Model Properties
From this little popup window you can see
Number of Vertices
Number of Faces

Please post both numbers in this thread.