Custom obj item models with no texture in the toolbar

Im having an issue where i can import my model fine and the items texture itself is there and everything works the only problem that in the players tool bar the item goes way out and shows the custom model as the texture . Is there any way i could fix that and replace the texture in the toolbar to something 2D or make it fit in the box i tired simply giving it a texture like a a slime ball but it still shows the custom model instead.