I haven't printed any big astro photos yet (haven't taken any good enough!
) but for my big panoramas, I send them to my brother on the Sunshine Coast. He runs a photo restoration business and gallery, and has links to a lab (I think in Brisbane) that can print up to 1m x 50m.
I stay away from printing myself. I know lots of printers can do a great job, but I haven't got one... so the local photo shop suits me for all my normal size and shape prints. Saves me the hassle!
Al.