The first number is width in millimeters (285) which is 11.22" and the second number is aspect ratio relating the sidewall height to the width which in your case is 6.17" for an overall diameter of 32.34"
I run the same BFG AT/KO in a LT275-65-R20 which is a 34" diameter tire with a 7.04" sidewall and 10.83" width on my 2012 1500 and am very happy with them. Stock suspension, no rubbing anywhere.
If you like the width of your tire you could go with a 285-65-R20 which is a 34.5" diameter tire with 7.29" sidewalls. Without a lift this is likely the largest tire you could run and you would really have to test fit to make sure you don't rub anywhere.