I whooped up some scale views to help you visualize. Each part is a different color to help you understand. The bench top (yellow) is 24x60". 1x6" skirts are in green and blue....
Front legs are 4x4, but the rear legs (due to attachment to the wall) can be 2x4. That saves some money. Here's a cut-away to help you understand the leg attachment....
See how the 20" deep skirt (dark green) in the rear comes down ? If you put one #8x3" sheet rock screw as high as you can get it, then another as low as you can get it into each wall stud, you'll end up with a huge triangulation arrangement that will support weight and make the bench super rigid.
The top is best made with a 4x8' sheet of good plywood, ripped into two 2x8' sections, glued to each other. But, you might cut material costs further by buying only 1 sheet of plywood. Whatever is not used on the top then becomes the rear skirt. The thinner top is reinforced by adding extra front-to-rear 2x4 bracing (red) standing on edge....
You can brush the entire finished bench with polyurethane or simple sanding sealer.
Legs are simply screwed on, allowing you quick disassembly if you ever move.