I remember I felt that the Book didn't deal in a very realistic way with the fact that a tiny town populated with few inhabitants, among them elders, women and children, couldn't handle together a destruction by a giant dragon and the sending of an army against an army of thousands of orcs.

Less than one hundred men in age to fight can hardly be called an 'army'.

Not only for Smaug, but also for the following, they had to scale up Laketown.

... which gives way to other oddities, such as : 'how can an town of 5000 sustain itself on a tiny lake without any agriculture around ?'

Tolkien never went into detail about the size of Esgaroth; his illustrations are the best clues that we actually have. But it wasn't very large. I haven't referred to anything (from Tolkien at least) as contradictory or illogical. I'm not sure what you mean with that comment.

Bard tells Alfrid, "It's a small town, Alfrid. Everyone knows where everyone lives."

In the type of world that is Middle-earth, as I believe you aptly pointed out at some point above in this thread, a city is not exactly a metropolitic NYC with a population of 17 million (or is it 19 million? I dunno- it's a freaking lot of people in a small area).

Nor is a town about the size of Bree necessarily considered "small". In fact, I'd venture to say that, given its location on a major, transcontinental crossroads between north and south, and east and west, Bree could easily be considered a city much like perhaps Denver, Colorado, US (a major airport hub). It isn't a small city, but it is no New York City either. I'd say Minas Tirith would be a good example of NYC, being the (temporary) capitol of Gondor during the War of the Ring.

I have a tough time equating what Bard says about "a small town" with what would actually comprise a small town in that sort of world.

In our modern society, a town of 5,000 is considered the backbone of rural America (my town is considered quite small with only 3 freeway exits, and it has a population of 27,000 as of 2010). We are quite rural- you can walk for perhaps half a mile in any direction except for towards the freeway (which we're a mile away from) and hit a farm interspersed between housing developments.

5,000 today is a relatively tiny town.

But in Middle-earth, there isn't the concentration of populace in such small spaces like there is today (excepting Gondorian cities like Osgiliath and Minas Tirith, which I mentioned above. I'll also note Hobbiton as being the central city in the Shire). I highly doubt that 5,000 in ME would be considered worthy of the title of "small town".

Esgaroth is a "port town". It supposedly is a town on a trade route, the "middle-man" between former Dale and Erebor to the east and south. But with Dale and Erebor abandoned by the time of the Quest, I imagine Laketown's losses from trade were staggering at best. Now they're at the END of that route, not in the middle.

If the town was so small that Bard can claim (without refute from Alfrid) that everyone knows where everyone lives, then I'd say that your guess of about 500 at most would be believable. That doesn't mean that the city couldn't have been more spread out. Doubtless there was enough of it that remained intact after the initial dragon attack on Erebor to increase it in physical size, if not necessarily population, to substantiate Smaug's attack in the film.

And I'm sure not all of the buildings were necessarily homes