In patterns where they give several sizes, say small-medium-large, you can subtract the number of stitches for the smaller size from the number of stitches in the next-larger size and get a number. Do this for each size larger, and you'll see a pattern. For example, maybe each size adds 6 stitches on to what the next-smaller size calls for.
Then you can take the largest size and add that differential [6 stitches in our example] to the stitches at that same step. This would give you, say, the equivalent of an extra-large. Want an extra-extra-large? Double the differential [in our example, it'd be 12 stitches]. And so on.
Just be sure to keep verifying the differences between the number of stitches at every step in your pattern - 6 stitches for casting-on, 4 stitches for armhole decrease, etc. - so you keep the same width dimensions.
And the length-wise measurements likely stay the same, of course.