Dispel Magic Only Ends Spells
A magical effect that is not the result of an ongoing spell cannot be dispelled.
Jeremy Crawford has issued several Twitter rulings to this effect.
In this Tweet:
Q: Can dispel magic undo a druid's wild shape
A: Dispel magic ends spells. Wild Shape isn't a spell.
and this one:
Q: Does Dispel Magic auto work vs Channel Divinity powers? Ie.. Paladin's Vow of Enmity magic effect.
A: Dispel magic ends spells. A Channel Divinity option like the paladin's Sacred Weapon isn't a spell.
and this series of Tweets:
Q: Does dispel magic effect an arcane ward?
A: Dispel magic ends spells. Arcane Ward isn't a spell
Q: Does that mean magical items cant be dispelled either? We've assumed that they could be disabled temporarily with dispel.
A: Dispel magic can end spells that come from a magic item, but it has no effect on the item itself.
Also note that Dispel Magic can only end spells with an ongoing duration. It cannot remove the lingering effects of an instantaneous spell. PHB P. 203:
Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.
The March 2016 Sage Advice Column covers Dispel Magic as well. It reiterates that Dispel magic cannot end magical effects that are not the result of a spell, such as a vampire's Charm effect. It also reiterates the rule about Dispel Magic not working on the results of a spell with an instantaneous effect. It adds that a readied Dispel Magic cannot be used in place of a Counterspell, though in some limited circumstances, it may be almost as good.
To answer your actual question, in this context, targeting a magical effect clearly means you can target just one specific ongoing spell effect instead of stripping everything on a particular creature.
The area of effect of dispel magic is already describe in the quote you used there: “one creature, object, or magical effect…”.
The point of confusion appears to be the idea that an AoE magical effect “must” need an AoE dispel magic to be properly targetted. But that's not the case: AoE magical effects are still singular magical effects and can be dispelled by a single-target dispel magic.
(Dispel magic would only need an “area” target description if dispel magic could affect multiple creatures, objects, and magical effects in a single casting, but it can't, so it doesn't.)
So if darkness was cast on a spot, it is an independent magical effect which you can target and dispel magic will affect the darkness spell itself. If darkness was cast on a creature, then you could target your dispel magic on the creature or the visible magical effect, but in practice you would always target the magical effect due to the difficulty of even knowing whether there’s a creature there to validly target.
Best Answer
No.
The familiar from find familiar isn't an ongoing magical effect to be dispelled. When you cast the spell (with a duration of instantaneous), the following occurs (from the spell's text):
The spell doesn't create the familiar, per se. Rather, the familiar is a spirit whose service you instantaneously gain when you cast the spell. So the familiar isn't a spell effect.
In addition, the spell's duration of instantaneous indicates that there is no ongoing effect to be dispelled (from the rules on spell duration):
So the familiar can't be dispelled using dispel magic.
For more details on dispelling magical effects, see this related question.