When you have “Labels” toggled-on in the sidebar (available for Grid Plots only), you can use the Cell Labels page of the Plot Options dialog to customize the label formatting.
Toggle-on “Include text box” to put cell labels in a text box. This can make the labels easier to see.
Use the Font and Color menus to customize the display of cell labels.
Use the Size box to set the size of the cell labels, expressed as a percentage of the frame. You can select a value between 0.1 and 8 for the size of the labels.