We recommend using 30252 labels on the DYMO 450. However, we also support using 30334 and 30321 labels.
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.