[Newbie] I had trouble with model import and export !

I had import model from Unity to Blender to created LOD for it, but when I export it back to unity I had a big issue with scale, it doesn't have the same scale with the original and it won't fit with the animation. Could you show me the best way to this? or how to fix it? I'm a programmer and I totally don't know about graphic design >.<!