You won't be able to find an O. s. under 2 inches. They are quite large when they settle out of the plankton. The smallest animal I've collected was around 5 cm. The smallest one I've seen in the industry is around 7 cm.
We know very little about their life history. There are no field studies that can be used to estimate growth rate, so anything I say about growth comes from the laboratory and is suspect. That said, we can make some reasonable guesses. First, in contrast to Gonodactylids that live in cavities and slow their growth rate as they get larger, O.s burrow and don't slow down. I suspect this is related to their being able to easily increase the size of their home. Using maximum lab molting rates and percent growth per molt, I guesstimate that a well-fed animal could reach 15 cm in 4 to 5 years. Maximum age is probably a bit longer, say 6 or 7.
Roy