Directory Opus 11 does not have full high DPI support. It was one of the major additions to Opus 12.
That said, you can configure Opus 11 to work fairly well in high DPI modes, but it won't be completely perfect in the older version and will take some extra configuration and tweaking, while things are pretty much automatic in the newer Opus 12. In Opus 11, you'll need to re-configure things for each machine independently if you transfer your config from a machine with one DPI to one with another. In Opus 12, you can transfer the config and Opus will automatically translate settings between DPIs. (e.g. If the thumbnail size is set to 256 on a standard 100% scaling machine, importing that config to a 200% scaling machine will automatically set the thumbnail size to 512.)
High DPI support is covered right at the start of the Opus 12 "What's New" video, if you're interested in those improvements:
Default thumbnail size is set under Preferences / File Display Modes / Thumbnails.
If you want it to be larger than 256x256 in Opus 11, you'll also need to adjust Preferences / Miscellaneous / Advanced: max_thumbnail_size. (In Opus 12, max_thumbnail_size automatically scales with the system DPI, so you would not normally need to adjust it, unless you wanted enormous thumbnails. Note that the larger it is, the more memory and disk space thumbnails use.)
That's unlikely to be due to 4k.
See Folder Formats: Quick Guide for a quick step-by-step on how to save new default folder formats for all folders.
If you want to specify fixed widths, make sure the auto-size columns option is not turned on on the format's Display tab.
FWIW, Opus 12 has some improvements to the way default formats are saved and also the way column sizing is specified, so that the auto-sizing settings are visible from the list of columns, as this confused quite a few people in the past. But you can achieve the same results in Opus 11 as well. (Opus 12 has some new column sizing options for different types of auto-sizing, which are shown here if you are interested: Directory Opus 12: Resizing File Display Columns, and Folder Formats.)
Short names are most likely turned off on the drive you are looking at. If the drive has them turned off, they don't exist and cannot be used to work around problems in software that requires them.
Windows 10 now defaults to turning off short names on drives it creates. They'll still be enabled on drives that already had them enabled, but they have to be turned on explicitly on new drives (or newly formatted drives). Microsoft have finally killed them, at least as a default setting, despite a lot of software (unfortunately) still needing them to work around poor programming. (Also despite them being useful to work around path-length limits which are still in parts of the Windows APIs themselves.)
The Short Name column in Opus displays what the OS and filesystem return when Opus asks Windows for the short names. If short names are turned off for a drive then you usually get back either nothing or the original long name in upper case.