I'm interested to have an idea on how the pka(s) of a compound is (are) determined and I would like to know what does macroscopic pKa stand for.

The are two kinds of pKa's determined- "true" thermodynamic pKa and what can be called a "functional" pKa. The former is measured making the assumption that as ionic components are diluted to infinity that activity coefficients approach 1. Often the calculation involves making measurements of the ionic components in some way (spectrophotometric, conductivity, electrochemical) at various concentrations. pKa is calculated at each concentration, a plot of pKa vs concentration made and a pKa extrapolated to "zero" concentration. The pKa at zero concentration(from the graph) is a thermodynamic valid pKa and is defined by
{aH][aH] / [aHX] = Ka
where a is activity for acid HX, anion X and proton activity. If one has a determination of pH, under the right conditions(dilute aqueous) the ratio of X and HX can be calaculated from this pKa. Alternatively concentrations and calculated values of activity coefficient can be used to get ratios of components or pH.

Often a functional pKa is calculated. In this case activity coefficients are ignored, or at least glossed over. So for example, if an equal mix of HX and X is prepared by mixing ingredients or titration, the pH of this solution will equal a "pKa" but since this pKa was calculated by glossing over activity coefficient considerations it is only an approximation of the true pKa, and only approximate values can be calculated from this kind of pKa.

If the solvent is other than water, then things get very complicated, but true pKa in some solvents can still be calculated. There is just more to worry about to get the right answer.

Hope this all helped as it is more typing than I usually endure.

PS I have no idea what "macroscopic" means in this context. Maybe it for strong acids that have big pKa.....ha ha ha...just kidding