To do this you can feed the regplot() function the scatter_kws arg like so:
import seaborn as sns
tips = sns.load_dataset('tips')
sns.regplot(x='total_bill', y='tip', data=tips,
marker="o", color="red", scatter_kws={'s':2})

sns.regplot(x='total_bill', y='tip', data=tips,
marker="o", color="red", scatter_kws={'s':20})
