To the OP - I'm not able to record the download size of the actual packages, but I can provide you the disk install. If you can meet the total size here, then you'll be okay.
I'll say you'll need at least 18-20GB-ish for the actual downloads
Healesville - 1.75GB
Cornish mainline - TRS19 - 6.92GB
Ecml Kings Cross - 3.68 GB
Cornish Mainline & branches (TANE) - 5.63 GB
Ecml Dundee - 5.32 GB
Total: 23.3GB
Cheers