If your lens is a 80mm CF, the filter size is B60. If you have an older variety 80mm or 80mm T*, the filter size is B50.
If you're using a B60, I recommend using 67mm filters with the adapter. These should help reduce the chance of vignetting if you end up using the filters with a wide-angle lens with a B60 mount.
58mm filters work well with the older B50 lenses.
I have an 80mm CF, and what I do is each time I buy a 67mm filter, I also get a B60 adapter that I keep attached to that filter at all times. This preserves the benefit of buying much cheaper 67mm filter, yet still having the fast mounting action of the Hasselblad bayonette mount.
I've had good luck with generic "made in Japan" adapters from B&H photo. I tried some even cheaper made in China adapters through eBay, and ended up returning those because of poor quality.