Since the early dawn of Dolphin, technology has advanced rapidly: Displays have higher resolutions and 4K became a standard. Especially for media designers a quick overview of images in adequate resolution is necessary. Unfortunately, 256x256 as a maximum preview size (as given in Dolphin as to date) for thumbnails seldomly satisfies. Yes, there are image viewers (gwenview etc) that can be used to see larger thumbnails, but imagine the use case of grandma trying to sort its private picture library. She has to switch back and forth between Dolphin and Gwenview, which is highly disrupting the workflow and not quite intuitive. So, my feature request would be A) Add an option to select the maximum thumbnail zoom size OR at least scale up to 512x512 for good measure and B) Add an option to save views per folder. Meaning: Have on default view (grid layout and thumbnail size), but when browsing pictures, make Dolphin remember the higher thumbnail preview size.
nemo and caja have a dconf setting for the thumbnail size that you can customize to any size you want. IMO this is a great solution because while I sometimes want very big thumbnails, I also understand that this is probably not a desirable option to provide in the preferences, as it may lead to performance or memory issues for some users, so having it as a dconf setting makes it clear that this is not an intended use and you're making changes at your own risk. Perhaps Dolphin could use some equivalent of that?
regardless I agree that 512 is reasonable these days. Dolphin went to great lengths to be the best for managing pictures by e.g. showing thumbnails of a folder's contents directly onto the folder's icon (something no other manager can do to my knowledge), so I think allowing for very large previews would go in that continuity.