I don't think there is a limit really... One, really hacky way to do this, would be too...
Position the camera at the entities position. Move away out. Render the model against a black backround, ensuring the model isn't black. Scan from the top, to the middle of the screen with readpixel commands. If only a few pixels are detect at the middle of the screen, then move the camera in and repeat. When a decent amount of pixels are read, you have a rough idea about the height of the mesh, based on how far out the camera is. Verrrrrry hacky way to do it tho...
Position the camera at the entities position. Move away out. Render the model against a black backround, ensuring the model isn't black. Scan from the top, to the middle of the screen with readpixel commands. If only a few pixels are detect at the middle of the screen, then move the camera in and repeat. When a decent amount of pixels are read, you have a rough idea about the height of the mesh, based on how far out the camera is. Verrrrrry hacky way to do it tho...