I would presume the bigger the dog, the deeper the bark. Not sure on decibel levels though as some small dogs are like babies and nerve shreddlingly shrill.

My Tibetan Mastiff has a formiddable bark and the Tibetans say it should be like a well made copper gong. I can confirm that is the case. If he's on guard duties, his bark is pretty deep and inside the house is loud enough to ricochet off the walls, make your ears ring your brain vibrate off the inside of your skull like a jelly on a vibrating plate exercise machine. Even worse than that is his "have I got your attention now?" bark, which involves him appearing behind me ninja style and screaming down my ear. That induces an out of body experience for a few seconds and a potential heart attack.

Following on from not being the tallest of heaviest dogs for a mastiff breed, the bark is not the deepest or the loudest, but it's more than fit for purpose.

Not sure if this has already been mentioned, but I have read the most common reason that Basset Hounds are surrendered to rescue is because of their bark.

Being a Hound breed they have been encouraged to 'give tongue' (euphemism for being noisy, lol) and although they are short, they are very deep chested (think sawn-off Bloodhound and you might be near the mark).... far from being a "small" breed they are a truncated Large breed.

When it comes down to noisiest, you need to think of breeds that are encouraged to bark for any reason versus size....

So anything heavy-bodied and broad chested and especially any hunting breed is going to be a bit loud.
Guarding breeds also.
Herding breeds less so (until you get into the herding/guarding types).
Many terriers are vocal as their handlers liked to hear them when underground to aid location.
Other hound types also vocal as they were bred to tell their handlers when they'd got a scent.

Virtually the only dogs that have never been required to be noisy are the sledding breeds - you can't bark when you're running, but you can make up for it later by howling...

And the sighthounds - you can't bark when you're chasing something at 200 miles an hour, and anything that isn't moving isn't worth talking about anyway....

My shih tzu x wakes the boxers if she hears something at night ..
Its what they were breed for i suppose ...

The loudest and most serious dog confrontation i ever heard and seen was between 2 bassetts at a show ...

My Cocker Honey has two barks - She has her deep bark for when someone comes onto our land who she doesnt know - She will bark at them and its quite a scary bark and quite deep.

We heard my brothers friends open there doors on our drive and honey ran up bark (she doesnt ever do anything, as soon as someone says something she waggs, but intruders dont know this) we hear them get back in there car and call my brother to come get them!! hahaha

Then she has another bark for when shes anoying out 13 year old springer spaniel Daisy and Honey wants to play and Daisy doesnt - She will just bark and bark in her ear while jumping on her head.. thats not even half as deep and is quite hight pitched.

I think it has a lot to do with lung capacity. When I started running my dog at 14 months his chest visibly got bigger and the bark immediately got louder.

A very fit and big GSD I knew used to make me jump every time it barked in the house.

im going to put asbo forward for the loudest!!! man is he loud!! his is quite high pitched too and it tends to echo in our living room... can hurt your ears sometimes

Benjie - high pitched kinda GSD bark, but not as impressive and so annoying because he barks at everything!

Sadie - JRT high pitched bark, happens when playing footie and window cleaners come.

Louie - really deep bark, totally unexpected for his cross, if he wasn't mine I'd be scared of him and a couple of Jehovah witnesses were scared off by it the other day..

So I'm not the only one who let's the dogs put off unwanted visitors ! They have their uses.

From my experiences at work (grooming):
Bassets have ridiculously loud barks. The deepest I've heard is a St Bernard that comes into work. Much deeper and more booming than my Estrelas, and they're pretty loud. The loudest barks tend to be the terrier breeds. Westies have a loud bark and tend not to want to shut up either!!!! Some of the most persistent barkers are Bichons, which tend to bark at a pitch that sets your teeth on egde! GSDs are pretty loud too. One of the noisiest dogs I ever had was a JRT. I had people coming off the street into my shop to see what I was doing to this poor dog. She was sitting with my friend - we'd tried her in a crate, out of a crate, on the table - to no avail she would not shut up. I believe the owners rehomed her soon afterwards because she was like that everywhere.