600-ish Size Quad

[Stoka]

So I';m relatively new to to the multirotor world, been flying a 330 since last summer and I';m now wanting to have a go building my own built for purpose.

Got a design in my head that I think will work but I';m stuck on a couple things. But here';s the specs I';m thinking of ;

650-ish size frame
NTM 2836 1200kv Motors
Turnigy Plush 30a ESC';s
10x5  Props
Naza M V2 W/GPS
Q-3D Gimbal

Now .. The arms I was going to make out of box section aluminum, what kind of size would I use for a quad like this and how would I go about mounting the motors?