I remembered a discussion about this from a long time ago. Here’s what I found in the archives:
Topic
File New – Size Custom Size is bizarre
robert stone – 06:18pm Oct 29, 2003 PST
When I click File New, the dialog begins wit a strange setting of 550 by 35 greyscale image. Can that be cahnged somewhere? Looked in eth Win2k registry but nothing found.
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Chuck Snyder – 6:22pm Oct 29, 03 PST (#1 of 1) Edited: 29-Oct-2003 at 05:41pm PST
Robert, the New dialog often picks up the dimensions of any picture or selection you may have recently copied to the clipboard. To get the default size, hold down the Alt key (Windows) and click on the icon for new file.
Chuck
p.s. If you do an Edit>Purge>Clipboard, you’ll get rid of the odd size and the regular File>New will open up the default size
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As to the part where you ask if there’s a way to change the default size, if the clipboard is clear, the dimensions you set will stay until you change them again. At least that’s how it’s currently working for me in PSE 2.