why don´t you use 18ct Aida? or 16ct... the higher the count the smaller the stitches (Aida, stitched over 1 Thread - evenweace or co you normally stitch over 2 threads, but also can stitch over 1 thread!
With few exceptions, you can stitch any design on any count of fabric -- higher counts will result in smaller images, lower counts are larger and more blocky. Things get even smaller when some (slightly crazy!) people stitch over 1 on 36 or 40 count. As others have said, the size of the design will be #stitches/stitches per inch, so its easy to calculate.
chekccard