If you are already using the AppInsight for Exchange template then you should be able to alert off the total mailbox size. I do not believe it is granular enough to alert on a specific folder inside of the mailbox.
As far as a custom powershell script, this would be the base of it. You'd want to add in error checking and validation stuff.
$Output = Get-MailboxFolderStatistics $MailboxName -FolderScope Inbox
Write-Output "Statistic.FolderSize: $($Output.FolderSize)"
Write-Output "Statistic.ItemsinFolder: $($Output.ItemsinFolder)"
In SAM, when you add a powershell component monitor, you'll paste the script into the Script Body section and then update the Script Argument section with the mailbox in question.
Once the script is tested once, it'll show the threshold settings below the screenshot above where you and set values for warning and critical.
This is an article that I wrote a while back that does into detail on the requirements for custom scripts.