I would agree with metengr, that the minimum fillet weld size would 1/4" unless a larger size is required for strength.
From my old "Steel Structures Design & Behavior" Salmon & Johnson, "fillet weld sizes are governed by the thicker of the two pieces being joined, except the weld size need not exceed the thickness of the thinner piece joined unless a larger size is required by calculated stress.
It is my understanding that the thicker plate draws heat away from the joint so a larger fillet is required to avoid incomplete fussion.
If the use of an 1/8 fillet to join a 1/4" plate to a 1" plate is now allowed, I have a lot of applicatons where I could save on welding costs.