Something to be careful of is that most of the formatting examples you are showing will result in a string. This means if you are using your techniques in a pipelined expression and pipe output to something like Sort-Object, the sort will be on the string which may not be accurate.
Here are other ways to format the number and retain a numeric value that will properly sort.
$size = 256654
$size/1KB
$size/1KB -as [int]
[math]::round($size/1kb,2)
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