Technically to be "tack" welder certified, a 2" long "tack" weld is required for the test.
So it could be considered a tack weld is to be 2"long. NowI'm not saying that's a hard and fast rule, but one could use that to argue the point. As far a size, I'd say 1/4" fillet minimum but again it really depends on the size, weight of the material.
Well,
The detail notation doesn't really give a person much to go by. I presume we are talking about standard fillet welds with the size and grade of materials you have and not any full or partial pen welds.
The only thing that I see that D1.1 has to say on the issue pertains to tack welds and rather or not they are being incorporated into the final weld. Tells you in Clause 5.18 that they must be welded following an approved WPS, meet VT if not incorporated into final weld, welded by a qualified welder, approved electrodes, etc.
D1.1 doesn't really give you an exact size and length on 'tack' welds. At least not that I am aware of. And I didn't see anything in it as I looked through just now.
Just my two tin pennies worth.
Have a Great Day, Brent