We recommend using 30252 labels on the DYMO 450.


http://www.amazon.com/DYMO-Address-Labels-LabelWriters-30252/dp/B00004Z64M


Update 12/01/14:  Additional label sizes are now supported via DYMO Direct Printing.  While in SETTINGS > LIST when you select your printer from the drop-down list, as shown in the video below, you can also choose the label size you want to use (30252, 30334, 30336 or 30321) from the second drop-down.