Font size for running text

Dear Kathryn,
sorry for the next question 😉 Is it possible to increase the font size for the running text? There are so many different css classes and everything I tried didn´t work ;( I just tried to expand the p style in line 151 of the style.css:

p {
  margin-bottom: 24px;
}

to

p {
  margin-bottom: 24px;
font-size: 18px;
}

But if I do it this way the blockquotes are also 18px ;(

Sorry for the next noob question, but I am more a graphic than a css person 😉