Converting input variable to Uint64

I'm having issues scripting the creation of a new VHD (A tool for the creation of network optimised packages). The script below basically pulls the total size of the input directory and passes that as a variable to the $intval function, which converts the size in bytes to a string, $size (nGB).

The problem I'm having is that the cmdlet NEW-VHD requires the -SizeBytes parameter to be in the format Uint64. If you enter the parameter manually, e.g.

NEW-VHD -path $vhdpath -fixed -SizeBytes 10GB

the cmdlet functions as intended and creates the VHD as it accepts the 10GB as a Uint64. What I need is the variable $size to be somehow converted to a Uint64 whilst retaining the trailing GB. Is there any way of imitating user input in this scenario?

I understand the below script isn't optimised or the nicest looking as it is only a proof of concept. Any suggestions regarding the above issue would be welcomed!

Code

$dir = Read-Host 'What is the directory you are wishing to store inside a VHD?'
$objFSO = New-Object -com Scripting.FileSystemObject
$intval = $objFSO.GetFolder($dir).Size / 1GB
$size = "{0:N0}GB" -f $intval
$vhd = Read-Host 'What volume name do you wish to call your VHD (no spaces)?'
$vhdname = ($vhd + ".vhdx")
$vhdpath = ("C:\VHD\" + $vhdname)
NEW-VHD -fixed -path $vhdpath -SizeBytes $size

I've had a look at a few Microsoft resources but have come up empty

Modified Code

$dir = Read-Host 'What is the directory you are wishing to store inside a VHD?'
$objFSO = New-Object -com Scripting.FileSystemObject
$intval = $objFSO.GetFolder($dir).Size
$size = $intval / 1GB
$vhd = Read-Host 'What volume name do you wish to call your VHD (no spaces)?'
$vhdname = ($vhd + ".vhdx")
$vhdpath = ("C:\VHD\" + $vhdname)
NEW-VHD -fixed -path $vhdpath -SizeBytes $size

Solution

Here is the code I wrote in order to get around the odd little bug.

#-------------------------------VHD CREATION-------------------------------------#

#Create a VHD with a size of 3MB to get around variable bug
        New-VHD -Path $vhdpath -SizeBytes 3MB -Fixed
#Resize to target dir + extra
        Resize-VHD -Path $vhdpath -SizeBytes $size
#Mount/Format and Recursively Copy Items
        Mount-VHD $vhdpath -Passthru | Initialize-Disk -Passthru | New-Partition -UseMaximumSize | 
        Format-Volume -FileSystem NTFS -NewFileSystemLabel $volumename -Confirm:$false
            $drive = gwmi win32_volume -Filter "DriveLetter = null"
            $drive.DriveLetter = "B:"
            $drive.Put()
        Copy-Item -Force -Recurse -Verbose $dir -Destination "B:\" -ea SilentlyContinue
#Dismount
Dismount-VHD $vhdpath