"50x50x50 cm box" - I'm a newbie in the 3D programming and unfortunately I don't know how to recognize actual models' dimensions, make a prewiew etc. I'm planning to realize this features, but I don't have any ideas now It would be really nice if you can give some hints about...

The getBounds method could have helped you but as you program in C#, you won't be able to reuse the classes of Sweet Home 3D. [:(]

I think that for OBJ files, you can guess easily the dimensions of the model: compute the min vertex and the max vertex by parsing the lines starting with a "v" and followed by 3 numbers (x, y, z) in the OBJ text file.