re: 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.