I found the font size settings in the ~\gaffer\0.59.0.0-beta\python\GafferUI\_StyleSheet.py file. Change `font: 9px` (line:201,214) to something bigger like this:
"""
*[gafferClasses~="GafferUI.Window"] {
color: $foreground;
font: 12px;
etch-disabled-text: 0;
background-color: $backgroundDarker;
border: 1px solid #555555;
}
QWidget {
background-color: transparent;
}
QLabel, QCheckBox, QPushButton, QComboBox, QMenu, QMenuBar,
QTabBar, QLineEdit, QAbstractItemView, QPlainTextEdit, QDateTimeEdit {
color: $foreground;
font: 12px;
etch-disabled-text: 0;
selection-background-color: $brightColor;
outline: none;
}