Remember; Tempest was not built from scratch. It's actually a vessel retrofitted over decades, a relic from 8 centuries ago dating back to the end of the Coalition-Alliance war. Coalition simply took the massive hull, and slowly upgraded it to Sirius technologies.
(as such, if Tempest is actually the Volgograd... where is the JiangXi, its sister ship?.... =P)
On flagship;
True, my original concept of a flagship is something large. Immense. A hugeweapon platform. Isn't this on our perception, might?
But to the book of modern warfare, size matters little. Really, an admiral usually won't choose his flag in accordance to firepower but will also consider all its strategical value, and sometime, a smaller, faster ship might just be what you really want.
For the Order, if they ever had a flagship, I'd think of a craft barely larger or maybe same size as Osiris.