To understand the size of site collection, you can use DiskUsed property in PowerShell console.  For example:

$site = Get-SPSite http://sp10dev
$size = (Get-SPSiteAdministration -Identity $site).DiskUsed
Write-host "Size of site collection is: "$size

 

If you use SharePoint 2010, you need to add before this code command "Start-SPAssignment  -Global" otherwise it returns empty value. Don’t forget to add "Stop-SPAssignment -Global" after code.

Start-SPAssignment  -Global
$site = Get-SPSite http://sp10dev
$size = (Get-SPSiteAdministration -Identity $site).DiskUsed
Write-host "Size of site collection is: "$size
Stop-SPAssignment -Global

 

To determine size of site collection in SharePoint 2010 you can use another property called "Usage.Storage". This property doesn’t require "Start-SPAssignment  -Global"

$site = Get-SPSite http://sp10dev
$size = (Get-SPSite -Identity $site).Usage.Storage
Write-host "Size of site collection is: "$size