Gerardo Zago Stefanoni, Thank you for posting this question on Microsoft Q&A. Based on your question, I assume that you have enabled the diagnostic settings on Blob service and have all the logs flowing to Log Analytics. In this case, both upload and download operations have the content size information available as well and you may use the query like below:
StorageBlobLogs | where OperationName == 'PutBlob' | project TimeGenerated, OperationName, CallerIpAddress, RequestBodySize
//RequestBodySize contains size as the file/blob is being uploaded
StorageBlobLogs | where OperationName == 'GetBlob' | project TimeGenerated, OperationName, CallerIpAddress, ResponseBodySize
//ResponseBodySize contains size as the file/blob is being downloaded.
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