I build my own equipment and sell hives also.
I use 3/4 inch thick lumber.
But be careful not all lumber is planned alike, some is 3/4 inch thick and some is 13/16 inches.
If using box joints use a caliper to get the correct thickness or the joints won't be the same lengths.
When making boxes cut all long boards 19 7/8 inch when using box joints and for 10 frame equipment 16 1/4 inches long for the short boards or for 8 frame make them 13 3/4 long.
Then depending on the type box, Deep, Medium or Shallow, 9 5/8, 6 5/8 or 5 11/16 inches
If cutting boards for Rabbit Joints make the long boards 19 1/8 inches long by however wide you want the box deep.
The short board will be the width of the box 16 1/4, 13 3/4 and so on inches wide.
On each end of the short boards cut a rabbit joint 3/4 inches wide and 3/8 inches deep or half the thickness of the board.
That will give you a box exactly 19 7/8 inches long by however wide you want, 10 or 8 frame box.
To make a 6 frame box cut the short board 11 1/2 inches long and a 5 frame box 9 1/4 inches long.