Do a search, and then from the search results page up at the top right next to where your sort order is indicated and where you see an icon to change from list view to gallery view, you should see an option for "customize". That should allow you to change your default search settings.
I have noticed that the customization does not always "take", particularly if you make many changes at once, so if you have trouble getting your new settings to stick, try this:
What has worked for me is to delete eBay cookies in my browser, then sign back in to eBay. Run a search and set up the view, sort and items per page that you want. Then go to customization and set the picture size and settings to include, and apply changes.
Reload the search and check if the defaults have been applied correctly. Make the necessary changes if not. After a couple of rounds of this, your default settings should be correct and not need any further changes.
