How can I select data size group by id for every value in a table
It seems that you need in
SELECT idMsg, SUM(LENGTH(txBlob)) AS total_size_in_bytes
FROM sat_msgBlob 
WHERE tmMsg BETWEEN '2021-03-01' AND '2021-03-02' 
GROUP BY idMsg

Then, after receiving the output into the client you may format it to Mbytes. Or divide the sum in the query to 1048576 and cast to decimal with needed fractional part length.