How do you check that a vector is so short that it should be considered as null? Like when calculating cross-product or scalar product of 2 vectors ...when do you consider the result null, so the 2 vectors are parallel or perpendicular? etc. Should it depend on the length of the input vectors?

Now I just check all 3 coordinates if all of them are between a -eps and +eps value. And what should this epsilon value be in general, ...that I use for telling if a number is "zero" or not? Should it be different for 4 and 8 byte float/double? Thanks!