Change initial upload data to be large and cut it off after X bytes, in a similar way as download size is calculated when downloading.

This issue is another optimization (number 6) for congestion control commented at #40130 (comment 2823615):

Note that also 20MB was just my test file value. It is probably better to make the POST extremely large (ie: as large as the largest possible sbws download is currently, or larger), and then just have a number of megabytes to cut it off at, after SS=1 SS=0. This number of megabytes can be calculated similar to how sbws chooses a file size for GET.