FS = Flowering Size. The plant can be expected (under good care that is appropriate for the species/hybrid) within the coming blooming season.
Note that FS doesn't necessarily mean that it's a large mature plant, it only indicates that it's large enough to bloom. Many Phals for example start blooming when they are still rather small, and are far from what people consider to be an 'adult' plant.
__________________ Camille
Completely orchid obsessed and loving every minute of it....