Auto adjust font size in seaborn heatmap

You can also do:

sns.heatmap(
    corrmat,
    vmin=corrmat.values.min(),
    vmax=1,
    square=True,
    cmap="YlGnBu",
    linewidths=0.1,
    annot=True,
    annot_kws={"fontsize":8}
)