Several new properties were added to GraphPlot objects in R2018b to enable customization of node and edge labels. In particular you can change the font size with the NodeFontSize and EdgeFontSize properties.
A list of all the properties is here: GraphPlot Properties
Example:
G = graph([1 1 1 1 5 5 5 5],[2 3 4 5 6 7 8 9]);
h = plot(G,'NodeLabel',{'A','B','C','D','E','F','G','H','I'});
h.NodeFontSize = 14;
