Dovetail joints are well suited to 3D printing, because most printers can print at 45º without added support material.
Here’s a good place to start when designing a dovetail. The 2.5mm width can be changed to suit the width of your part at the joint. I recommend keeping the 3mm depth for any size part. It is strong enough to hold all of the parts I have tested without breaking the mating part, and requires a small enough area to not require drastic redesign of the mating part. I also recommend keeping the 1mm trim, to prevent your slicing software from attempting to make a corner that is far sharper than any normal size extruder can make.
When fused filament printers print, the extruded molten filament pushes against the previous layer, which causes it to extrude sideways (need a drawing here). This makes the parts a little bit wider than the CAD model, even on a perfectly calibrated printer. Holes will be smaller in diameter, and pins will be larger in diameter.
To make a dovetail slot for mating with the dovetail positive, you need to add some clearance. 0.25mm all around seems to work reasonably well for most printing orientations on my printer. Try one and adjust the clearance to something that works well for you.
Here are some test parts I designed for checking clearances of dovetail joints. I designed these parts to demonstrate the differences in clearances created by printing the same geometry in different orientations. Note that none of the joints run horizontally along the side of a part. That type of dovetail cannot be printed without support material.