Th KB/MB/.. in the result list or the status bar is 'just' the presentation and is independent of your search query.
You still have to specify size:>8KB
Note that in Everything 1.5 you can specify what standard to use for sizes (see link), as 1KB can be 1024 bytes or 1000 bytes, depending on which standard is used.
That does have influence on your searches, so size:8KB is either size:8000 or size:8192
In the interests of less keying, I would like to suggest a parameter similar to the "Size Format" option for displayed results is provided to set the default unit in the search bar.