it will be fine! i dont personally think the "king bettas" are that much bigger then a normal betta. some people think it could be a giant betta, if it is though, it will still be fine
i had a "king plakat" from petco in a 10g and it was fine.
as for the tank, the fluval edge might not be a good choice, since there isnt any open air, and bettas are labrynth fish, and they frequency suck air from the surface