It is "common" practice to include a general note such as: "The leg dimensions of all fillet welds shall be equal to the thickness of the thinner member unless otherwise specified by the welding symbol." The intent is that a fillet weld symbol without the size specified is covered by the general note. Another note I see is: "Unless otherwise noted by the welding symbol, all fillet welds shall be 5/16 inch." Again, the general note covers any fillet weld symbol where the weld size is not specified.
By "common practice" I don't mean that it is required or recommended, just that I've encountered it on a number of projects. This flies in the face of D1.1 for materials greater than 1/4 inch, but you didn't reference D1.1, you referenced D1.3.
Best regards - Al