Where to start?

The drawer size will dictate your apron width. I would add about 1" to 1.5" width of the drawer. You also need to account for the legs width on both sides.
If you are making a square table then that would be the size for all four.
If it’s rectangular then the side aprons will be shorter.

For the top I would take the apron size and add about 1.5" all around.

Since you have the height, assuming your top thickness is 3/4, the length of the legs would be total given height minus the 3/4 thickness of the top.

This said, I have seen some where the drawer take all the front with no space between it and the table leg.

Also I am no expert take all this with a grain of salt. I have done only 5 end table all different styles.

Start with what you know. Think about the drawer and the components you’ll use to build it. That will give you the outside dimension of the drawer, which will dictate the opening. From there you can size out the apron that will be proportionate to the height and style of the table as well as how the legs will attach. Another factor at that point will be any hardware used for the piece as that can also alter the design somewhat. Once you have the main box designed, determine how the top will blend with the piece and then look again at the legs to make sure their lines are in keeping with the design you’ve got so far.

I have a client that has brought me drawer pulls and asked me to design a piece around them. Fun project!

Bill:
Excellent advice.

MJCD

Thanks guys. For a 21"W 15"D 3"H drawer, what guide system would be best? I was thinking a center guide on the bottom.

MontyJ:

It depends on use…, and several aspects thereof: Will it hold anything heavy, or will result in a lopsided weight distribution (side-mount roller cage); do you need deep access (full-length slide); will the drawer be frequently used (roller bearings, side-mount)? Short of these, good old wooden slide – basically a glued-on tenon sliding loosely in a wide mortise (dado) would do it.
MJCD

MontyJ, try a square table, use square legs but learn how to mortise by hand. Use pine. You can cut 2×4′s for legs on a table saw. And, get some really grade-A 3/4" shelving board with no knots and laminate your top. Use a hand-plane to joint. But mortise the legs, and tenon the aprons. And, don’t forget to consider expansion/contraction when you mount the tabletop. Just rabbet the drawers, leave the dovetails for another build.

Thanks MJCD. I should have included a few more details. The drawer will receive light to moderate use. It’s propose is to hold fabric cutting mats, so nothing heavy or shifty. The entire table will be designed to hold the sewing box I finished several days ago. I think I have a pretty good idea now of what I’m going to do.

David, I appreciate the advice, but a square table just won’t work for this project. This particular project has two purposes. First, build a table to hold the sewing box. Second, challenge myself with new techniques. I’m a “jump in with both feet” kind of guy. I’m not going to back away from it because it’s hard. I’m going to do it because it’s hard (well, hard for me anyway).