Yes, this is possible. Before importing an Excel document, do the following:
Create a custom palette section (see "Edit" -> "Manage Palette").
Create a single node of the desired type and adjust its label font size to your liking.
Add the aforementioned node as a node style template to your custom palette section. (Right-click the node to open its context menu and choose "Add to Palette".)
Double-click the corresponding template in your custom palette section to make it the default template for new nodes.
When importing Excel documents, switch to tab "Presentation" of the Excel Import settings and make sure option "Template" is set to "Selected Template from Palette".