Using 18 count fabric will just make the pattern smaller.
You can work out how big the pattern will be very easily:
Count the number of stitches to the pattern, first left to right, then divide by 18 - that will give you the measurement of the design in inches. Then do the same for the number of stitches going top to bottom.
For example, if your design is 36 by 18 stitches, the design will measure 2 by 1 inches on 18 count fabric.
If you're not happy with the size of the design, you could always stitch a border. There are lots of free pattern designs on the internet, or you could design one yourself.
Happy stitching!
Cat.