Wow, I cannot believe there has been no other replies to this post. I wish I could be of more help to you. I am new to this hobby as well. I have only had one urchin which was the purple pincushion. But I do know from a friend of mine that has a pencil and it is huge.
I really don't know how to know the size off the site. I am from Canada and cannot order from the site but "I think" when they give the size say example 2-3" fish I think that is the size it will be when it is shipped to you. I don't know for sure though.
Another thing you can do is a search on google and there are lots of other sites that come up regarding what you are looking for and some of them give the full grown size of things.
Also, post the question here. Say you want a particular fish, post the type of fish on the forum and ask what size the fish is when full grown. Lots of info from members as well as the search bar too.
How much live rock do you have in your tank?