How to set flag ship?

Just purchased my first proper 55-man longship but when I enter the battle map I'm still on the rinky dink 27-man starter!  How do I set my flagship so I can raid in style?

Click on "See Ships", there should be a number beneath the name of your ships, or the word "flagship" for your flagship. Click on that number to move it up in the list, with the first ship being the flagship.

I have a related question: Can I remove individual ships from my fleet, without selling them (leave them in a different port)?

And also: why oak-made snekkja is so "fat" compared to a ash-made one??!