There are lots of ways to do this. Here’s one:
switch -Regex ([math]::truncate([math]::log($bytecount,1024))) {
'^0' {"$bytecount Bytes"}
'^1' {"{0:n2} KB" -f ($bytecount / 1KB)}
'^2' {"{0:n2} MB" -f ($bytecount / 1MB)}
'^3' {"{0:n2} GB" -f ($bytecount / 1GB)}
'^4' {"{0:n2} TB" -f ($bytecount / 1TB)}
Default {"{0:n2} PB" -f ($bytecount / 1pb)}
}