Dogs will happily eat a variety of raw vegetables and the same is true of broccoli and cauliflower.
I usually chop it up roughly into 1cm cubes and then mix it in with their main meal.
Occasionally I will give them a much larger chunk of broccoli or cauliflower but since they are already used to these vegetables I do not have to worry about them becoming a choking hazard.
It might be best to stick with smaller chunks if your dog prefers to inhale their food rather than chew it.
When your dog spits out raw vegetables it is best to steam them for a few minutes.
Some dogs prefer when there is less crunch when eating broccoli or cauliflower steamed as it does not destroy too many nutrients.
If you want to read more about dog food tips read here: Dog Food Tips and Tricks.