One needs to accept that Find Familiar (Lvl1: Wizard) is very powerful, RAW does not prevent familiars been ordered to travel large distances. The instructions are always understood and an attempt will be made to deliver an object if the location and path of the target are known.
- Only a single tiny object could be allowed.
- Under most conditions, there will be a significant chance of failing to deliver the object unless the familiar has been previously instructed to memorize the target location and the route is safe.
- Once the object is dropped at the location, there are no warranties that it will be found by the intended recipient.
Here I try to respect RAW and balance whenever possible. The main limitation that allows balance are the intelligence and knowledge necessary to find the target location, the events that may interrupt the travel and the likelihood the object will not be found once delivered.
There is clearly a lot in common between this question about balance and "Q76171: Can a familiar travel any distance?". While Q76171 only asks if it's at all possible, this question assumes it is possible (based on RAW "always obey your commands") and focuses on asking if it is balanced, and what distances to allow.
On the answers to Q76171 the consensus seems to be in accordance with the RAW, a familiar can be given a command to travel (a familiar always obeys your commands) a large distance. The 100 feet limitation applies only to telepathic communication.
To answer the question about balance, and the conditions and distances to be allowed, one needs to address the following issues:
- Is the command intelligible to the familiar?
- Does this replicate the effects of Animal Messenger (Lvl2: Bard, Druid, Ranger)
- Can the familiar reasonably succeed on the task?
IRL untrained animals do not understand commands from people, in D&D5e familiars are spirits that "always obey your commands" so the message should be received by the familiar without a language barrier, thanks to the magical link between the caster and the creature.
However, familiars act with the stats of an animal of the chosen form, so intelligence (usually 1-3) will limit the likelihood of success. Consequently, this is one opportunity to limit the effects to maintain balance.
A DM could rule that familiars of low intelligence could not recognize specific people, only places.
The familiar must know the way and the place to attempt to deliver the object. Known places but unknown routes or conditional statements will most likely fail. The Familiar could get lost and be unable to find the location, if unfamiliar with the location or the path to it.
Recalling a location could be facilitated if the spell caster linked to the familiar instructs their familiar to memorize a particular location and associate it with a name. Otherwise, the item may en in any random location in the general area.
Complicated orders may be attempted but will always fail.
Replicating animal messenger?
No, this does not replicate Level 2 spell Animal Messenger (Bard, Druid, Ranger), on which
[The message replicates] the sound of your voice. The messenger speaks only to a
creature matching the description you gave.
The "sound of your voice" is not replicated, a "creature matching the description" is not found magically, and no new animals get recruited to serve. This is a spirit familiar already linked to the character which is following an order. Furthermore, there is a class mismatch, Find Familiar is a Wizards' spell (And subclasses that use Wizard spells like Arcane Trickster Rogues) while Animal Messenger is a spell for Bard, Druid, or Rangers, even if the effects were to be the same, within a class, there is no duplication of effect for a higher level spell.
The target travels at the speeds defined in the statistics of the chosen form. It could travel at a maximum speed rate and does not have to stop at regular intervals to rest or feed because it is a spirit, not a beast.
You gain the service of a familiar, a spirit that takes an animal form
you choose[...] the familiar has the statistics of the chosen form,
though it is a celestial, fey, or fiend (your choice) instead of a beast.
A "familiar has the statistics of the chosen form" and is therefore susceptible to the limitations of that form. They are susceptible to being attacked, affected by weather or getting lost, or being obstructed. This could imply that the familiar may be "killed" (disappear) or forced to drop and lose the message with a likelihood proportional to the distance travelled.
Whenever the familiar drops to 0 hit points or disappears into the
pocket dimension, it leaves behind in its space anything it was
wearing or carrying
This is the other main opportunity to limit the effects of this idea to provide balance without conflicting with RAW. At the discretion of the GM, the Familiar could be interrupted and fail to fulfil the instructions.
Consequently, the distance itself is not a limitation, but the time it takes to reach the destination is a limitation. Also, the likelihood of being interrupted on the way must increase proportionally to the distance and the difficulty of the terrain.
Other sources of failure
The spell caster may miss-calculate when is safe to recall the Familiar, and that could happen before the message is delivered.
Once delivered to a location, the object may never be found. Probably dropped in an inconvenient location like on a tree, roof or well.