Max wheel size for '03 AWD Astro?

I want to purchase some 18" rims for my '03 AWD Astro van but not sure what the max wheel size I can put on is? Stock size is 215/70/16 with a diameter of 27.69". My choices are 245/50/18 with a diameter of 27.64" or 235/50/18 witha diameter of 27.25". So what's the max I can  put on there?

i just put wheels on my astro and you an fit up to 29 inches tall will rub a little. 28 in should not rub