A sawbench is primarily used to support wood for ripping or crosscutting with a handsaw.  The main difference between a sawbench and a sawhorse is the wide top.  They are also usually much shorter than a sawhorse, though some people make short sawhorses for assembly.

The dimensions can be somewhat arbitrary, but the height of the sawbench should roughly correspond to the distance from the floor to the bottom of your knee.  In use, one leg is bent at the knee and placed on top of the workpiece, holding it secure on the sawbench.  If the sawbench is too tall, your other foot cannot rest flat on the floor.  If the sawbench is too short, your other knee will have to bend too much.

I used some of the old 2x4 and 2x6 boards left over from building the workbench.  With the circular saw, I ripped a couple 2x4’s down to 2–1/2″ wide (by 1–1/2″ thick) for the legs.  The remainder of the sawing I did by hand.  I laid out a 10° kerf in the miter box to cut the legs.  Then I sawed, chiseled, and planed notches for stretchers.

I made notches in the top to receive the legs.

The tops of the legs were also cut back.  Then I chamfered the edges and countersunk for screws.

The screws and notched joints hold the sawbenches together, no glue necessary.  I made a V-shaped ripping notch on one of the sawbenches; a traditional feature, though I don’t know if I will actually use it.

I am short, so my sawbenches are only 16 inches high.  This height can have an effect on practical handsaw length.  The first determining factor is the length of your arm stroke, but a short sawbench may limit you to an even shorter saw.  In the second photo below, taken while sawing the ripping notch, you can see that in a steep ripping angle even a 22-inch saw is almost too long.  Any longer and there is a risk of hitting the floor with every forward stroke.