ATM it's based on a dollar value and whether or not you opt for a tab large. Large is the only one that requires not only you meet a minimum dollar threshold, but that you also pick a tab large plan. If your plan is at least 40 dollars, or 35 in Quebec, then you can get a tab medium (360) without changing your plan. Tab small (240) is only 25 dollars a month.
If you are on the old tab system still, that limits what you can see.