method to determine the fillet weld size

It's called design. Are you just looking at the min and max size in the AISC and picking one?

Before attempting structural design I recommend you take an in-depth course.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

Jor1492:
Those types of tables are not a good way to design welds. Welds are designed as a function of the loads imposed upon them, the weld metal tensile, yield and shear yield stresses, and upon the strength of the base metals. Those mins. and maxs. in fillet size have to due with welding process, heat input to make a good weld, to much heat for thinner piece, ability to actually fit the weld to the pl, edge without melting nicks into that edge/upper corner, etc. The min. weld forces a large enough weld to cause sufficient heat input to allow the weld process to take place without cooling so quickly that cracking takes place or no real fusion takes place. The max. size pays attention to how large a weld the thinner pl. can tolerate, heat input, deformation, etc.; and it also recognizes that you can’t really get a full .5" fillet on the edge of a .5" pl. without a bunch of potential problems or extra work.

I hope you have actually been deigning your welds. Just picking the max. one out of the table doesn't mean it will have sufficient capacity. The table doesn't know the loads in your structure or the weld geometry etc.

Hi jor1492

You need to design the welds to withstand the operational forces the joints see in service, have a look at this site:
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein