As I write, I can see the water lilies on our courtyard pond - I just had to draw them...... Worked in pure cross stitch with minimal backstitch and no fractional stitches. This could be stitched on linen or Aida fabric. It would be very effective if you added some Balger blending filament to some of the petals to add that natural glisten.
To work out the design size (or the stitched area) you need to divide the stitch count by the number of stitches to the inch (2.5cm) on the fabric. You then add enough fabric to this measurement to allow for framing etc. If a stitch count is not shown on any design, you will need to count across the design and down the design, which will give you the stitch count.
Example: 140 stitches divided by 14 (Aida) = 10 inches
If working on evenweave fabric, remember that you generally work over two fabric threads, thus 28-count linen is also 14-count when doing the sum. I suggest adding 5 inches to the design size on a project to make sure you have an adequate margin for mounting and framing. Obviously you can adapt this amount when working a small card of lid top.