Overhead - you can use 5 cubic inches per conductor, the grounds only count once, this leaves you with the need for 35 cubic inch box. Now, let's take into account that you are making joints on a circuit that is more than 30A, and most wirenuts aren't rated for more than that. This leaves you needing to make joints with either crimp splices (needs a special tool), split bolts (kearneys, needs lots of tape, including rubber splicing tape), or you can get a bigger box, mount some isolated tap blocks and go that route.
Personally, in the interest of keeping it as simple as possible, I'd use a 6x6x6 box, and kearney the joints. Get an electrician to help you do this part, if you don't do it right, you'll have one hell of a fireworks show.
If it's underground, first of all, I'd avoid making joints. If you still insist, get the underground splice kit from 3M and still make everything up in a J-box. (at least a 18x18x6 PVC weatherproof box)
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