As per me, there is no table or formula for maximum fillet weld size. But then the question arises why should the fillet size be too much more than the minimum design or code specified weld size. When the leg length is doubled, it is worthwhile noting that the area is increased four times. Unlike a groove welded butt joint, the increase in area and thus weld volume being very high in a fillet weld, the cost of welding, the consumable quantity and associated problems like distortion would be much higher when increasing the fillet weld size. Of course one point in addition is that the minimum fillet size when specified on a WPS in accordance to AWS D1.1 is only for the multipass. For the single pass, it is the maximum size that is specified. The minimum and maximum in the case of multi and single pass is obtained by actually measuring the leg lengths on the Procedure qualification coupon.