Tires:
the first number is the width. The second is the heighth and a PERCENTAGE (iirc) of the width. Meaning... your 100/100 18 is, in theory, as tall as it is wide. So going to a 110/100 will give you a slightly larger diameter, but I don't know that anyone would notice it near as much as they'll notice the extra width.
To make matters a bit more complicated... some companies measure the width of only the casing, others measure at some other spot (knobs?) which reads wider. I run Michelin Cross Competition in the front (it's an MS2 or S12, not sure) and an S12 rear. My rear is something like a 130/80 which is the same as your 110/100.
I know it's odd. The point is, you can go a bit wider and be fine.
-Dawg