Here is a neat merge layer trick from Cobra405, one of the moderators here at Aviary.
"First off, select the layers you want to resize and merge them. Then, with the merged layer selected, go to Select>Select all. Next use the Edit>Copy function to copy this layer to the clipboard. Now here comes the clever bit :) Hit undo twice. This will recover all of your layers as they were before you started! Finally, just use Edit>Paste and hey presto, you now have a new layer with the merged layers you copied. You can resize all you like, and you also still have all of your original layers!"
Now that is what I call brilliant. Thanks for the tip, Cobra!