Lactobacillus are smaller rod shaped bacteria compared to the cyanobacteria of oscillatoria and gloeocapsa.
That depends on your definition of 'nucleus'. The nucleus or core of an atom is, obviously, smaller than the atom as a whole. But seeing as you placed this question in biology, a cellular nucleus is...
Since it is in the cell it has to be smaller than a cell. I am guessing it is probably about a thousand times smaller than the cell.
Organelles are to cells are organs are to organisms.
About a quarter.