I think the largest size you could put on is 235/75/16. If you go on a tyre conversion site it will tell you how much bigger they are and the difference in speed. Hope this helps.
I reckon the nearest alternative on a 16" rim would be a 235/60 16 which has a circumference only 10mm bigger than the 215/65 16.
I would be careful of a 235/75 16 as it could well foul the front suspension.
Its also a good bit bigger - 2" in diameter bigger - so your speedo would be well out.
Dunc's suggestion to try a tyre conversion site is sound advice.
215/65R16 is 27" overall diameter (standard).
235/60R16 is 27.1" overall diameter (0.1" or 2.54mm bigger than standard).
235/65R16 is 28.03" overall diameter (1.03" or 26.16mm bigger than standard).
235/75R16 is 29.88" overall diameter (2.88" or 73.15mm bigger than standard!!).