I doubt that this Lysiosquillid is 15". The only species in this family that gets that large is Lysiosquillina maculata and while animals up to 38 cm have been reported, it is rare to find one over 32 cm. Atlantic species don't get over 27 cm. I can't be sure from this photo, but the color suggests that this is a female. Females are not a spectactular as males because they have reduced raptorial appendages.
These are not difficult animals to keep and you could easily hold it in a 60 gal tank. I've posted instructions several times on how to construct a suitable tank for one of these guys. The biggest problem is that very large Lysios cannot produce sufficient mucus to construct a sand burrow. Therefore you have to either keep them in a substrate with some "stickiness" to it , a problem since this probably means organic material, or in a pvc burrow. This animal would probably be o.k. in a 3.5 or 4 inch diameter pvc pipe at least 50 cm long. Better yet, try the cut-away design that I have described.
Roy