Sure, locate the style.css file within your theme and paste the code below. I suggest using percentages in your declarations so as to keep the base size styling hopefully already in your theme.
h1 { font-size: 150%; }
h2 { font-size: 140%; }
h3 { font-size: 130%; }
h4 { font-size: 120%; }
h5 { font-size: 110%; }