To increase the size of an individual gene set node, double-click on the name of the gene set in the results table on the right hand side. Each time the gene set name is double-clicked, the size will increase. Since the appearance of labels depends on the size of the node, this is a way to add labels to specific nodes in the network.
To increase the node size across the whole network or just for specific node types, please use the top menu Node => Size dialog. As shown in the Fig. below, in a bipartite network, you can change the node size for only genes (keep gene set nodes same)