Go as big as you are able. Coastals climb. They might get bigger than 3m under your care. Forget the minimum specs. They are worthy of 5-star accommodations. If you can, make that cage a showpiece for your masterpiece.
I have an 11-foot Olive in a 1828(wide) x 914(deep) x 457(tall), because he never showed any interest in climbing. That seems to fit him with room when he stretches out.
BUT, a few months ago I got a 2014 Coastal. I plan on having a significantly taller enclosure with at least a shelf when she gets big.
If you can, go big.