Junction boxes come in many shapes and sizes. Depending on what sized wire, how many wires are in a given box and the number of wire splices in that box, you may need a little more room than a standard box can give you. You may have to opt for a deeper or larger box to accomplish the splicing you have planned.
When choosing the right sized box, consider the size of wires that you will be installing in the box. Obviously, if the wires are #14 or #12 non-metallic sheathed wires, there should be more that enough room for a few of them in a standard 4" box. However, if you have a few #6 wires in the same box, it can become very crowded quickly. I this case a 4" deep box, or better yet, a 4 11/16” box. It gives additional room width-wise and is a deep box.
What's the Difference in Box Sizes?
The box on the left, in the picture, is a standard 4" box. It is 1 ½” deep box. On the right is a 4 11/16" wide box that is 2" deep. You can see how much more room this box provides for splicing electrical wires. By allowing ample room in boxes for spliced wires, you eliminate the possibility of pinching a wire between the box and cover plate.