What does Nominal Size mean?
The following definition of nominal size is from Wikipedia: "In manufacturing, a nominal size ... is a size "in name only" used for identification. The nominal size may not match any dimension of the product, but within the domain of that product the nominal size may correspond to a large number of highly standardized dimensions and tolerances."
Some manufacturing and construction products come in nominal sizes instead of the actual size. You have most likely heard of this regarding 2 x 4 lumber. The actual dimensions of a 2 x 4 are 1.5" x 3.5".
Nominal sizing is very common in tile, in particular, ceramic tile. One reason given by manufacturers is that ceramic tile shrinks during the firing process.
Regarding my medallions and backsplashes, all sizes / measurements are the actual size unless noted in the description next to the size. I use nominal sizes on some of my products to match the size of the tile used in that product. For example, 18" Daltile Chamber Cliff Straw and 18" Daltile Santa Barbara Pacific Sand are labeled with nominal measurements. The actual size of those tiles is 17 5/8" x 17 5/8". So, when I craft a medallion using these tiles, I will usually craft it to the same dimensions as the actual tile: 17 5/8" x 17 5/8". Of course, as with any of my products, you can custom order to the exact size you desire.