ok i have a mesh in blender. so i created another messh in the same window. its almost the same shape. but when i load it in nifskope and add the collision mesh. its smaller than the actual mesh. any ideas

When you export meshes from Blender, there is a Scale factor that you can set as one of the many options in the menu. Make sure the scale is 100% so that it matches the mesh you imported. You can also set the scale when Importing meshes, so you need to mind both when importing a model to modify and exporting it again. There shouldn't be any other reason why a truely-duplicated mesh would have a different size as far as I know..

if you did any scaling of the mesh in blender, it'll retain that information in the transform matrix, which will get exported if you don't zero it... so perhaps this is a collaspe the transforms of the mesh before export?