Very little advanced info is available. I think that's a deliberate part of Gustin's business model; they want you to keep coming back to their site to see what's new. They also don't want you to pass on an item because you know something else you may want more is coming up next. If you sign up for their email list, and follow them on Facebook, Twitter, and Instagram, you'll sometimes get a small amount of advanced notice of new campaigns.
Regarding sleeve length: I'm a slim Medium in most shirts, and Gustin's Medium has the perfect sleeve length for me. You should be able to figure out where you stand by measuring one of your well-fitting shirts and comparing with their Fit Guide.