Print downloaded corpora directory sizes in download_fuzz_corpora.py

Print the sizes of downloaded fuzzer corpora directories on disk after
downloading and unzipping them. To be used for identifying
oversized/duplicated corpora on code coverage builders.

Bug: 503931690
Change-Id: I1dcca4158d028dab9175b670e392c51f7c96d6ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7904358
Reviewed-by: Haiyang Pan <hypan@google.com>
Auto-Submit: Daniel Saldaña <dansal@google.com>
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Commit-Queue: Daniel Saldaña <dansal@google.com>
Cr-Commit-Position: refs/heads/main@{#1642502}
NOKEYCHECK=True
GitOrigin-RevId: 80f202f2de209560a96d915beb927af131518242