Hi,

Please try add more this code to Customize > Custom CSS to do that:

.post-entry blockquote, .post-entry blockquote p, 
.wpb_text_column blockquote, .wpb_text_column blockquote p{ font-size: 18px; color: #ff0000; }

Change value 18 and color code ff0000 to any value as you want.

Best Regards,

PenciDesign