Waterdeep is approximately 3.8 miles x 1.5 miles
Or ~20,000 feet north/south and ~8,000 feet east/west. Both measurements were done along the greatest straight-line dimension of the city. This measurement is done by measuring the space between the walls of Waterdeep, so it includes the enclosed section of the harbor to the south.
This was derived from the map provided in the 3rd edition book City of Splendors: Waterdeep, which actually includes a scale. A copy of this map can be found here if you would like to make more precise measurements of distances between things.
I checked other sources that relate to Waterdeep, such as Volo's Guide to Waterdeep (2nd Edition), and did not find any contradicting maps. The 2nd Edition adventure titled Waterdeep includes a map, but no scale. The 1st edition sourcebook Waterdeep and the North does the same...map, but no scale.
Thus, with no contradicting evidence, it's probably a fair assumption that the 3rd edition scale of the city is accurate.
However your table wants
Is it more usual to ask the other players 'out of character', or is it more common to roll some kind of in-game check to identify the creature?
Yes, it's usual and yes, it's common. And unless you poll every group that's ever played (or at least a statistically significant sample - PhD anyone?) you won't know which is more frequent. Look, some people like to create deep and complex worlds where each session is a new revelation and some people just like to chop things into mincemeat - you do you, ok.
Metagaming
I hate that word.
For a start it has two meanings:
The game about the game. Looking at the character classes and choosing to play a ranger rather than a wizard is a metagame decision; just like a coach watching video of their opponent in this week's football match and consequently adjusting strategy is metagaimg. This usage is not problematic and is usually not what's meant when a role-playing gamer uses the term.
Particularly in a role-playing game; it's applying knowledge that the player has that the character ostensibly doesn't have. It's this usage that is problematic and I will now indulge in a short rant because the concept sets my teeth on edge.
Its a game of make-believe elves in a made-up world using rules that cannot be anything but the most abstract model of non-reality!
I cannot for the life of me see that rolling a d20 to see if you "hit" is not metagaming1 but claiming that your character, who supposedly grew up in a world where vampires are real, knows that sunlight hurts them isn't. Hell, vampires aren't real in this world but everyone knows your best defense is a garlic-wrapped wooden stake!
OK. Rant over.
Basically, it's offensive metagaming if it breaks your table's verisimilitude and it's ok metagaming if it doesn't. So, you and the people you play with need to set your own boundaries on this, just like you need to decide how naked you can be while you play.
Options
- It can be fun to find out by trial and error. For the DM, yes, this is a laugh riot. For everyone else, it can be an exercise in frustration. I've sat on both sides - don't do this.
- Finding out by being clever. This is good for the DM and the players. The DM places clues. The players find the clues. The players put the clues together. The DM and the players feel clever. Feeling clever is fun. For example, the Spear of Osiris in The Mummy Returns - while this was incredibly clunky for a movie it is 100% perfect for a role-playing game because players don't have scriptwriters to tell them the answer. See the three clue rule.
- If the player knows the character knows. This rewards experience and, in most games, people with more experience are better at the game than those with less. Why not in role-playing games? How does the character know? Who cares. Or, see below ...
- They just know. I know the difference between a lion and an antelope. This is amazing because I don't live in Africa, have never been to Africa and have never hunted/been hunted by either. How can I possibly know something about the creatures that I have never encountered? Books, stories, myths, legends are all things that exist on Earth - why aren't they in your fantasy world?
Of course, you can and should mix and match - trolls are common so everyone knows about fire, but kdsja2 are rare, so only the most arcane tomes and learned sages know about their vulnerability to tulip bulbs.
1In the second sense - it clearly isn't in the first sense because that is the game.
2Don't bother looking for kdsja, I just made them up.
Best Answer
There's nothing about the geographical features of the map that could be used to objectively determine a “right” scale for either map.
So instead, decide practically: pick two points on the map and decide how long it would take to travel via a given type of transportation. Work backwards from that to determine the distance. Measure that, and translate it to a map scale.
For example, if you decided that travel on foot between Ramor and the crossroads just north of that little person icon in the second map takes half a day, and in your world travel on foot is at approximately 24 miles per day (include rest stops, etc.; see DMG p. 242), then the distance of that section of road is about 12 miles. At ~47 pixels, that gives us a scale of about 0.25 miles per pixel. That makes the continent with Ramor on it about 195 miles north-south, 270 miles east-west, or 330 miles on the long diagonal. To add a scale to your second image, then, draw a line about 40 pixels wide and label it “10 miles”. Then scale up for the world map: since the zoomed-in continent is about 1305 pixels diagonal and it's about 309 diagonal in the world map, your maps have a scale ratio of about 4.22. So on your larger map, the scale marker could be 95 pixels wide and labelled “100 miles”, assuming that you decided that original measurement takes a half-day on foot.