The sun puts out about 1000 Watts per square meter, or about 0.1 Watts per square centimeter.
The human eye has a blink reflex of about 1/4 of a second.
Maximum permitted exposure related to a light source equivalent to the sun, and when the pupils is constricted to about 2mm in diameters is a little under 2 Watts per square centimeter.
So you probably want an organ to be able able to produce that much light intensity at some distance lets say about 10 ft or 3 meters, if the diameter of the spot was about 20cm, then pi r squared give the need to illuminate about 300 square centimeters or about 600 watts.
That might be a problem if the creature needed to produce 600 watts continuously. For reference a human at rest is about 100 Watts in just the thermal energy it radiates. A resting human probably needs about 1400 calories for its base metabolism. To compare: 600 joules per second* 86400 seconds per day divided by 4184 Joules per calorie would mean the creature would need about 12,390 extra calories per day just for its light source. Thats about 30-40 ham sandwiches.
However, to dazzle, if it is pulsed light that needs to be only 1/4, maybe 1/2 a second, each light pulse would only need a fraction of a calorie.
The next problem is the efficiency of getting the light out of the creatures body. The sun's rays are parallel by the time they reach the earth, and that is important since that helps the dazzled creature eye focus the light onto the dazzled creatures retina. If the light is being produced in all directions inside the creatures body, then most of it is going to be absorbed in the emitting creature's body. That might be o.k. but it might need some kind of reflective coating around the organ, and that could be tough compared to a photoreceptor and even be able to take some heat build up per pulse. Since it is a fraction of a calorie, we could say the creature might use up a ham sandwiches worth of calories per pulse and still be o.k. if the creature didn't pulse too often.
Now to answer the question about the size of the organ needed.
The nice thing about bioluminescence is that is is about 96% efficient and little waste heat is produced.
1) If it is the bacteria themselves, I suppose you could try to have them coordinate to all flash at the same time. That could be stimulated by a chemical signal, or perhaps telepathic signal by the creature.
2) If the creature extracts the needed chemicals from the bacteria, it could have a more concentrated biochemical light source, but would need some mechanism of keeping the component chemicals separated. However this might have the advantage of being much more efficient per volume.
With approach 1 there are issues in having good ways to feed them, keep them alive, but also the energy density is probably a factor of 10 or perhaps 100 x less than mixing the component chemicals. If it is approach 2 the chemicals there probably needs to be some kind of storage, and specialized membranes to purify and keep the chemicals from reacting and tissues to mix the chemicals when needed.
From an Encyclopedia Britannica for luminescent materials
The energy required for excitation therefore ranges between 40 kilocalories (for red light), about 60 kilocalories (for yellow light), and about 80 kilocalories (for violet light) per mole of substance<
Assuming a two types of molecules with a total atomic weight of 100, (100 grams each per mole) that would react and excite different molecule to be excited for another 50 to 100 grams that that would release the photons. That might be about 300 grams for the chemicals. The for the system perhaps as much again in for grams for the tissues, that would be about 500-600 grams which is about the size of a large human fist. You might make it a little bigger for optics, or the cavity, and make it longer to have a better telescope or something. This assumes the reacting molecules all react quickly, but there might actually be a longer glow after the chemicals were mixed.
There are probably some bad chemistry assumptions but for a ball park without getting into the details of the molecules and how they react etc. it might give a rough idea.
Since volume goes as the cube. if the chemical method was about 10x10x10 cm, and the energy density of the biocells was 100 times worse, you need a volume of about 46x46x46 cm.