A few things:
1) Try disabling your antivirus/firewalls while downloading. There have been a few issues with them causing the "file larger than expected" and other installation hangups.
2) Try blue orb > preferences > folders > Reset Data. This also fixes the "file larger than expected" errors sometimes.
3) Sins has a lot of files. It can take a bit to verify. The new Impulse patching mechanism is a lot better than it used to be, but depending on other things, 15 minutes may not be too long.
4) You can access the Impulse error log by clicking the question mark in the top right. It would help the Impulse devs if you posted it along with your issue in the Impulse boards, as kryo suggests 