I am trying to figure out what size junction box I need to connect 6
awg wire.

I went to my local hardware store and the person there looked at me
like I was an idiot, after talking to them for a min I realized they
were the idiot...

Anyways, the biggest box they showed me was a 4x2 1/8 square box, which
I know is just too small. Not enough room to join a 3x6 awg wires.

Basically I have hooked up my hot tub (which is outside) using armor
cable for the outside run then when I ran out of that wire I bought
another 30ft of indoor wire. Obviously the junction box where i am
butt joining the wires is indoors. So basically I just need to know
what size junction box I should be using.

Mighty Miindy