Want to folder size in sharepoint online as a column or any easy way ( i am user not admin)

Hemant2004 
If you want to compare metrics before and after migration please be aware that SharePoint modifies some file types during uploading. For example, an msg file will typically get 5 kB larger because SharePoint injects schema information. Office files (docx, ..) are also affected and even tiff files.
You can compare the number of items of course. That should match before and after.
There are browser-based tools that provide reporting capabilities (example) and generate a csv file with all nested content (including path, type, sizes, modified dates, encryption, etc..).
Another approach is to use PowerShell scripts. For example here.
Or perhaps use the OneDrive for Business client. Do not sync all documents to your local system but simply use the OneDrive tool to get the properties of the folders/files present.
Paul