How to change the marker size in lmplot

You want to use scatter_kws={"s": 100}

As in:

lm = sns.lmplot(x = "totalX", y = "NormI", hue = "Data Type", data = df, palette="Set1", legend_out=False, scatter_kws={"s": 100})

You can amend the integer value (currently 100) to change the size of the markers.

I don’t know what your hue or palette data are, but this should work nonetheless.