How much is too much or too little for donut spare?

I've got a space saver spare for my car that is .25 smaller than my rear tires. I haven't used it yet but in the event of a flat I will use it to get my car to a place I can repair my flat. I'm planning on driving as short of distance as possible and at slow speeds. The rear axle will think I'm taking a really. Long curve:-)