1) Determine weld pattern and weld size required at cap-column interface to resist the horizontal force (shear).
2) Sizing the plate, so it covers the column with edge distance for welds (weld size + 1/16"), and a width at least equal to the beam width.
3) Determine the number of bolts, and the bolt size, to take on the shear.
4) Work out the plate thickness by checking the bearing strength over the bolt holes. Round the number and add 1/16" tear and wear.

Note that occasionally you will run into a situation that everything turns out very small, then use code specified minimum, or minimum size(s) set forth for the project.

Do you have any uplift / tension force from the beam acting on the column?
If so, you will need to check the flexural strength of the plate due
to the tension / bolt tension.