Use the Theme with Larger Fonts
The theme with smaller fonts is called normlg*.css.dsp. You can configure WEB-INF/zk.xml to use it as follows.
<desktop-config><disable-theme-uri>~./zul/css/norm*.css.dsp*</disable-theme-uri><theme-uri>~./zul/css/normlg*.css.dsp*</theme-uri></desktop-config>