...Primary factor deciding arm length is size of the prop, then the purpose as for FPV you will need longer arms so that the props can be out of camera view.Or because of space requirements or frame size...
Forgot to mention it I'll be using a 10 x 4.5 prop and no FPV but I would use it for aerial photography. I'll be using carbon fibre for the arms.
I need it to be stable I won't have any problem even if it is large in size.
...first time I heard about "tri's arm length double a quad's arm for the same stability"...
Don't remember it clearly where I read it, maybe read it a week or two ago, maybe in an rcgroups.com topic.