Honestly if all you are putting in there is esc's 6mm is the size you want.
how thick are your arms, 3mm? get 5mm so you have 8mm of space. put a m3 bolt right through an nut on the other side. mine and many others are like that, my esc fit fine.
10mm would work too, any bigger is over sized.
Use hollow ones and bolt from the other side, you can't thread it then in a crash.
First videos from my first flights
First videos from my first flights
Nothing special, got my mate on the gimbal controller, we have some awesome plans though once I get a bit more confident and experienced
Not sure iv asked these before but have you got motor updrades on there? what escs?