Dera Sir, According to AWS D1.1(6.12.1.1): Round indications such as porosity not lees than 1/4 in(6mm) when the material thickness is greater than 2 in, the maximum acceptable size porosity is 10 mm. Best Regards. O.Khodabandeh
You identified D1.5 as the applicable code. Figure 6.8 addresses discontinuities in tension welds. The way I interpret the nomograph is that slightly more than 3/8" would be allowed for the maximum size (part way between 3/8 and 7/16").