Size of the X and/or Y axis labels

[CRETE Denis]

Hi,
I tried many different ways to increase the size of the text labelling the x and y axis of a graph. Among them:
xtitle('','T [K]','Ic [mA]',font_size=3)
which does not work. Does anybody know the command to incrase the font size of 'T [K]' and 'Ic [mA]' in a script (and not with the graphic properties editor, which will only work "manually")? 
Thank you in advance
Denis