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Well, I'll start with the obvious and point out that small world's don't have to be satellites, they can be planets -- Ganymede (Jupiter's largest moon) and Titan (Saturn's largest) are both larger than Mercury.
As for tidal locking (the effect that one side of the Moon always faces the Earth, or one side of Mercury always faces the Sun), again there's no requirement that moons be tidally locked -- most moons in our solar system aren't. Tidal locking generally requires a relatively large secondary body (Mercury, the Moon) to be relatively close to its primary (the Sun, the Earth) -- if the Moon were either smaller or further away from the Earth then it wouldn't be locked.
Lastly, you may want to reword your post a little for clarity -- it's a bit unclear what you're saying in places as it seems like you're using "planet" to refer to both bodies, particularly in the penultimate sentence.
There is no 'dark side' of a tidal-locked moon, the term 'dark side' is just poetic words for 'side of the moon that never faces the planet'. That side still gets sunlight when it faces the sun.
Not to shoot out your tires, but I've found lunar phases and such are more important to GMs than to players, most of which are happy to be told, 'oh yeah, it is a full moon tonight', and otherwise not pay much attention.
Seasons are a function of the orbital tilt of the body, not whether or not it's a planet or moon. The half of the body that is tilted towards the sun is in summer, away in winter.
That is unless you want to make an extremely elliptical orbit for your planet, then the distance from the sun will influence things.
Either way, being a planet or moon will have little relevance on the seasons.
Your bigger concern is that if you are making the world the moon of a large body, then your day-night cycle is going to be messed up, as the reflection of light from the primary will likely be bright enough to keep things well-lit. Your world will only have true night when it's orbit takes it on the opposite side of the primary, so "day" and "night" could be several days long each if your primary is large enough, as in Jovian sized.
Thinking about it, things would probably get really weird as the moon of a Jovian. As you would have full day when the world is on the day side of the primary and facing the sun. Then varying degrees of day-night as the world is facing away from the sun, but still lit by the waxing and waning light reflected by the primary. Then full night, possibly several days of it, when the world is completely eclipsed by it's primary.
Looking at the major moons of Jupiter and Saturn, the orbital period ranges from one day to 79 days. You could have some fun with a undead campaign with a regular full night that lasts a week or so.
A small planetoid/moon would have difficulty sustaining life without an atmosphere. Thus, seasons would be irrelevant--unless of course there was an artificial means for attaining said atmosphere.
You could make the moon any size, and just limit your amount of landmass. Or areas that sustain life, like only near the equator, or near the poles. To most players and especially their characters, the exact shape of the world isn't an issue or perhaps even a known fact.
As for life/atmosphere; it's magic or divine, unless you wish for it to be realistic(as opposed to fantasy).
To have an atmosphere, size is less important than magnetic(or other) shielding and mass(often related to size, but really more about composition).
Note: Depending on distances, when the moon is between primary and sun: your nights would be bright(if the primary gives off a least some light) and the moon could cast its shadow on the primary - don't forget to give that dark spot a name :)