Just a quick module with a function to parse size unit. For example: Parse-SizeUnit "1GB" or "1 GB" will return 1073741824. Improve support up to YottaByte.

SizeUnit.psm1

function Parse-SizeUnit {
    [CmdletBinding()]
    param(
        [Parameter(Mandatory=$true, Position=0, ValueFromPipeline=$true)][string]$SizeUnit
    )

    $SizeUnit = $SizeUnit.Trim()
    
    $Size = 0;
    $Unit = 'B';

    if ($SizeUnit -match '\d+\s+\w*B$') {
        $SizeUnitSplit = $SizeUnit -split '\s+'
        $Size = [int]$SizeUnitSplit[0]
        $Unit = $SizeUnitSplit[1]
    } elseif ($SizeUnit -match '\d+\w*B$')  {
        $SizeUnit -match '\d+'
        $Size = [int]$Matches[0]
        $SizeUnit -match '\D+\w*B$'
        $Unit = $Matches[0]
    }

    if ($Unit -eq 'YB') {
        $Size * 1PB * 1GB
    } elseif ($Unit -eq 'ZB') {
        $Size * 1PB * 1MB
    } elseif ($Unit -eq 'EB') {
        $Size * 1PB * 1KB
    } elseif ($Unit -eq 'PB') {
        $Size * 1PB
    } elseif ($Unit -eq 'TB') {
        $Size * 1TB
    } elseif ($Unit -eq 'GB') {
        $Size * 1GB
    } elseif ($Unit -eq 'MB') {
        $Size * 1MB
    } elseif ($Unit -eq 'KB') {
        $Size * 1KB
    } elseif ($Unit -eq 'B') {
        $Size
    } else { 0 }
}