I have finished a beautiful task of getting the individual sizes of site collections present in each Web Application.
Here is PS cmdlet, which has to be run, as shown below.
- Get-SPWebApplication <Web App name> | Get-SPSite -limit all| Select Url, @{label="Size in MB";Expression={$_.usage.storage/1MB}}| Out-File -FilePath "C:\Output.txt"
You will get the output in a txt file in C Drive as per the path provided in PS script given above and again put over it in an Excel sheet.
I have faced a challenge here that the whole URL and size in MB got clubbed in a single tab and unable to separate it.