Assuming we are working with AWS A2.4
The weld size for a square groove weld is what is placed in parentheses on the left. This is typical for partial penetration groove welds.
If the square groove weld is to be CJP, than it is more clear to use the "melt-thru" designation on the reference line opposite the square groove symbol. If the weld is CJP there is no requirement to place the weld size, as it will be the thickness of the base metal.
See AWS A2.4 7.2.2 and 7.2.3