I have experience with 3 different dane knots, received anally. Honestly the size of the dog was more of a problem than the size of the knot. Dane's are very awkward because they don't seem to understand how large they are. Mine have always been 170 pound lap dogs. I've speculated on why you don't see many dane videos, and what I always seem to find is two things:
1) Dane's aren't that entertaining or photogenic. Due to their body size and energy level, they don't really hump for long and when they do all of the action is on the inside and doesn't make for a great video. The feeling of their GIANT knots is indescribable, but you also can't see inside the hole to see just how big they are, or how much they pulse when they're cumming.
2) Yes, the size. This isn't just the size of the knot, but the whole penis. The ability to handle a knot as large as a most dane's anally requires some extra preparations, cleaning, stretching, etc. Vaginal penetration is easier in many ways, but also more restrictive. The vagina is better suited for accommodating something of that size than the anus. However, a vagina's depth does have a limit especially when a knot is also inserted. The anus can handle much longer objects as well as the knot.
For reference, my last dane was my largest. He was about 27cm / 10.5in long in total. His knot was around 9cm / 3.5in long and would grow to approximately 10in / 25cm in circumference.
The experience I've had with ALL of my boys, my danes and every other dog I've been with, is still special to me. So yes, when I'm able to have dog(s) again, I will be very happy to spend much of my time locked to them despite how large, awkward, smelly, or painful it can end up being.
