Specifically which elements do you want to make bigger? The menu text, the entry title, the meta data (author & date), the article content?
The default font size is 12px. You can change this by adding this rule using the theme’s custom CSS option (if it has one) or by using JetPack’s Custom CSS option (go to Jetpack → Setting and activate Custom CSS, and then you should see an Edit CSS option under the Appearance menu):
body {
font-size: 14px;
}
This increase the current default font size from 12px to 14px. You should see an immediate change in the size of the article content. If you want to change the size of other elements, you’ll have to add additonal rules for them.
