Answer 1
When it comes to Halong Bay cruises, 'biggest' and 'luxurious' are not synonymous.
The most luxurious junks tend to be the smallest, but if you like crowds and karaoke then go for one of the very big ones.

Answer 2
have a look at Bhaya- they have 3 or more different sized junk boats and all three are gorgeous.
although I have not been on Emeraude, and i haven't heard much about it lately, and It is not a traditional junk, but it is big and has a good reputation as a 'luxury' boat. it's more like a 'paddle-steamer'