it depends on local fishing regs, for the uk the regs are on the tide timetable. for anything even slightly worth it i would go for a carapice [body] above 10cm personally.
Shore crabs that are near me grow 9 centimeters max. But I'll go ahead and give it a try anyway. I've made one of these http://www.bushcraftuk.com/forum/showthread.php?t=13958&p=185607#post185607 and once I've tried it (assuming that I catch something)I'll upload some pictures.