By default, ShipStation will print labels in a 4"×6" format suitable for thermal label printers. If you use an inkjet printer to print multiple labels on a standard letter size sheet, you can change the output format.
Head to your Account Settings (the gear icon in the upper right).
Next, select Printing and then Printing Setup from the sidebar on the left.
Here, you'll see the different documents that ShipStation can print. Clicking on the Document Options link under Options will give you formats and preferences for printing each document.
Select the Document Options link for Labels and change this to the appropriate format for your printer and preferences.