@Zareen under Validation & Defaulting, you can set a maximum length, to enforce an upper limit of 9. For a minimum length (lower limit) you might need a customization.

Got it, do you know what kind of customization I would be looking at?

You can certainly do validation with a client script or user event script, either of which would be my choice. It may be possible to do with a workflow as well, but I am not sure of that.

I wouldn't do it with workflow, would be kludgey at best