It'd be nice if there was an easy calculation to optimize windows on the south face, but there are many variables.
It is a trade-off between introducing additional heat load in the summer to be removed, and introducing additional heat load in the winter to reduce heating requirements. Of course you want the incoming sunlight to reach an internal thermal mass so the heat can be utilized throughout the daily solar cycle, but without that mass there is still free heat during daylight hours. The presence of mass would permit you to increase the solar exposure, as you can store the peak heat during the day.
The size and orientation of overhangs also affect the magnitudes of the additional load in summer and winter. However, solar angle maxima and minima don't precisely correspond with heating/cooling load maxima and minima, so this effect isn't a simple calculation.
Excessive direct solar exposure can be discomforting, as it will cook you, essentially. I suppose ideally the light is directed onto a thermal mass and that mass heats the space & occupants indirectly. This leads to concepts like the trombe wall. Although it's nice to have the view afforded by the windows, and a giant wall in front of the windows doesn't help that! I suppose I'm preaching to the choir here lol.
Anyway, I found a free program
RESFEN good for selecting window areas and LowE coatings for a given building and its climate. I can't testify to its accuracy, but its by far the best free option available I've found.