You can change the font size for a MATLAB legend by setting the 'FontSize' property of the Legend object.
For example, plot four lines. Create a legend and assign the Legend object to the variable 'lgd'. Then, use dot notation to access the 'FontSize' property and set the value to 14 points.
plot(rand(4))
lgd = legend('One','Two','Three','Four');
lgd.FontSize = 14;
Alternatively, you can specify the 'FontSize' property using a name-value pair argument in the 'legend' command. When including name-value pair arguments, include the legend labels in a cell array. For example:
legend({'One','Two','Three','Four'},'FontSize',14)