The 2 main labels that are used by FedEx are the LS-U (7.5×5-1/8″ label) and our LS-6x4BL (6×4″ label). You could print the labels 2up or have the entire sheet as the label and combine a packing slip on the bottom portion, both of these options would require special software.
If you want to print the labels 2up, we have our LLS-8.5×5.5-2up. The one we use in our order fulfillment is ShipRush for FedEx Shippers.