There is no way to properly give a rule of thumb for weld size.
The weld size should come from the designer of the equipment, as it depends on the force in the weld. To develop the strength of a plate, you need a fillet weld approximately 1.5 times the plate thickness (assuming you are only welding on one side only and the strength of the weld metal matches that of the base metal). So by arbitrarily selecting a weld size equal to the plate thickness, you may not be providing enough weld in some cases, but in other cases, you may be providing a lot more than is required with the consequent problems of increased welding time and distortion.