First, set_xticks does not have fontsize argument because set_xticks sets the location of the ticks, not the labels to be shown. Those can be set via set_xticklabels, and indeed set_xticklabels does have a fontsize argument. However, it requires you to specify the ticklabels, which only makes sense when using a FixedLocator.

ax.set_xticks([1,4,5]) 
ax.set_xticklabels([1,4,5], fontsize=12)  

Usually you would neither set the tick positions nor the labels to any fixed numbers but use the default locator and formatter or use your own locators or formatters. In those cases the easiest way to set the fontsize is to use .tick_params

ax.tick_params(axis="x", labelsize=12)

It does not allow to set other font or text related parameters like font-weight or alignment though. This is indeed a missing bit in the API. A slightly less convenient way is to use pyplot here

plt.setp(ax.get_xticklabels(), fontsize=12, fontweight="bold", horizontalalignment="left")

which then allows to set all properties at once. The drawback is that it is kind of convoluted and requires pyplot.

In conclusion I think there is some need to change the way text properties can be set on axis labels, but set_xticks is surely not the right place to do that.

As written above, set_xticks sets only the positions and thus does not handle labelsize. Use set_xticklabels or tick_params.

赶紧加上fontsize 寻思啥呢 大兄弟