What size junction box is needed for four 6 AWG copper wires?
Any (exterior) type you like, with a volume of 20 cubic inches or more. 5 cubic inches per #6 wire.

Here's a calculator http://www.constructionmonkey.com/calculations/electrical/boxfill

You may find boxes  bit larger than the minimum make your life easier, or are needed to make reasonable connection to your conduit size, etc.

Look up "figure 8 coiling" before you install your wire - if you can't get enough people to have one standing by (each of) the middle box(es) while you pull it end-to end, pull from one box to the next and figure-8 coil the wire on the ground - then flip the coil over and pull the next segment (no tangles.)