set fontsize <value>
The setting fontsize changes the size of the font used to render all text labels which appear on graphs and multiplot canvases, including keys, axis labels, text labels produced using the text command, and so forth. The value specified should be a multiplicative factor greater than zero; a value of 2 would cause text to be rendered at twice its normal size, and a value of 0.5 would cause text to be rendered at half its normal size. The default value is one.
As an alternative, font sizes can be specified with coarser granulation directly in the latex text of labels, as in the example:
set xlabel '\Large This is a BIG label'