What is the right size of compression sack for the Black Diamond Hooped Bivy Sack (Long), or does the hoop make a compression sack impractical?
Hoop is tough to compress. You can probably do it, but I'd worry about the wear and tear. Bibler used to (and I assume still does) include a reasonably-sized stuff sack. It gets the bivy down to ~12 in by 5in, as I recall.
15 x 48 cm, 6 x 19 in (Long)