According to NEC 314.28, pull boxes for conductors 4 AWG or larger in a straight pull must have a length of at least:

• 12 times the trade size of the largest raceway entering the box
• 4 times the trade size of the largest raceway entering the box
• 8 times the trade size of the largest raceway entering the box
• 6 times the trade size of the largest raceway entering the box