I have dealt with questions like this in both directions
Your question is a little different because you did not give anything about the target MySQL Instance (what Storage Engine will be used, which columns are TEXT/BLOB, what indexing is needed for your data, etc.)
IMHO the most straightforward way I can think of is the following
Write some Perl/Python script to sum the length of all XML tag delimiters. Subtract the sum you get from the length of the file. That would essentially be the size of a CSV file less delimiters. This would be the bytes of the raw data.
Do METHOD #1 and also take the count of all XML tag delimiters and add that count to the result of METHOD #1. That would like adding commas back into a CSV file. This make be closer in size to an actual CSV should you print the output to such a file.