So I will chime in, using this as an opportunity to appeal to skinmakers new and old.
I know that for many this is obvious, but also sometimes we can forget about something....
When making new skins/templates for existing aircraft (as daddyairplanes said - using as big as possible format 4096x is quite good enough). But before starting a new skin/template make sure, what needs to be done for that model.
Many new aircraft (but also old ones) have bump maps (normal) and Specular maps as additional features - that can improve the quality of the model as a whole. So when you are making new templates in higher resolution - make sure to make new bump maps too* - so they will correspond with new skins. It is not (never) going to look good if you have nicely done 4096x skins with beautiful thin lines, and small rivets if you will combine it with - let's say stock 512 bump maps you will have a mess with lines and poops instead of rivets.
Interesting is a fact that bump maps can be - 'bumped" and "sunken" at the same time - just a matter of arrangement and creativity. - for example depressed lines and raised rivets etc...
The same rule applies to Specular Maps. And this is somehow neglected more often than you can imagine, especially with metal/silver skins. Not going to dwell on why specular maps are used (or not) but since they are used sometimes they also have to match skins. Personally, I love it when a model can use specular maps, as it can give nice depth to the skin. And again wrongly made specular maps can spoil the final effect
And specular maps are bringing one more issue. If those are assigned to a model then they should be used always in a specific skin folder despite the skin being silver/natural metal or camouflaged. If a model is using specular maps they also need to be attached to a camouflaged skin folder. Otherwise, the skin will look pale, or too glossy, or too shiny, or simply not the way it should.
So no. Specular maps are not only for shiny metal surfaces. Not to mention, that with some practice create nice "different" surfaces.
while bump maps should be at least the same size as all skins to have the best quality (4096x or 3072x) and must be inside the main aircraft type folder, specular maps usually can be much smaller (even 4x smaller - 1024x1024, as for some small pieces of model like fuel tanks, extra rails if any specular maps can be even 512x512. And it must be (again) in every single skin folder.
* when one has done new templates already - making bump maps is an additional 10 minutes, and it is somehow easy with certain software.
examples of mismatch between skin-bump maps - in fact it is hard sometimes to portray it on the screenshot, but hope you will get the idea.
skin 2048x2048 mixed with stock TW bump maps
and here bump maps based on that new skin set 2048x2048 (by ludo)
wrongly edited specular maps. This is an extreme example but shows how it should not be.
clearly you can see us air force "brand" on the fuselage and just near roundel additional roundel "underneath"
noticeable is the reddish canopy - because I used a specular map from different skin sets where the canopy is painted red. This can be used to advantage when we want to add sometimes something special to the skin.