I believe the one thing the star chart establishes beyond doubt is that we are looking at Sector Z-6 and its quadrants (reminder: in TOS a quadrant was a smaller piece of a sector!).
On the other tentacle, if the title of the chart is "Earth Outpost, Sector Z-6", we are left wondering why only a single outpost is mentioned when the map shows seven. If, instead, the chart reads "Earth Outpost Sector Z-6", then we are very probably speaking of terminology developed specifically for designating Earth Outpost Sectors, rather than for general astrography!
I'd also be wary of considering the "5000" thing a scale bar for the map. There'd be very little point in providing a bar that is 2½ grid sides long and doesn't even nicely divide into an integral number of tick marks per square! But if it really is 5000 units per 2½ grid sides, then it's 2000 units per grid, which is amusingly suggestive of the 20 lightyear length of a sector side in the TNG backstage model of things...
Naturally, having each square here measure 20 ly per side is a possibility. But I'd rather go with these being quadrants within a sector, as the map supposedly depicts a "sector", not "sectors". For us users of the decimal system, a 20 ly cube side sounds odd, but whatever rationale led to choosing such a weird number may also have resulted in choosing some multiple of 20 as the quadrant side length. We're then probably talking about a smooth fraction of a lightyear as the basic unit here, that is, these quadrants are defined as having 2000 of a unit derived as a "lower order of magnitude" of a lightyear - 1/100th, 1/1000th or somesuch.
Related musings: after opening credits, Kirk says that his ship was already on a mission of "patrolling outposts" when summoned to help OP4, yet he has to rush to the map area from the lower left for some reason. What does that tell us about the layout of the outpost chain? Kirk is closest to OP1 initially - does this mean he was merely on his way to start a grand tour of the chain when the SOS came? OTOH, how could a simple "wall" going from lower left to upper right and starting at OP1 hope to contain the Romulans, if Kirk and other UFP assets are to the lower left? Surely there must be OPs below the map area as well...
If so, do these have negative numbers? Or is the OP just below OP1 perhaps designated OP 1882, the other end of a circular chain featuring 1882 outposts? Where's the three-dimensionality in the setup?
We could rather say that OPs 1 through 8 (or 9 or whatnot) are designations for outposts in the Earth Outpost Sector Z-6, while Z-5 and Z-7 to "left" and "right" have their respective OPs 1 through 9, and Y-6 and A-6 to "up" and "down" likewise sport ten outposts each; such a setup could cover a vast volume of space, allowing for the multi-star-system RSE, and the "two-coordinate" nature of sector names would support the three-dimensionality of the setup.
A general note on scaling the thing, too: We get a lower limit to RNZ width from those episodes that feature a star system within the Zone itself, and
then we get "The Enemy" where Romulans enter the RNZ from their side and say to their own man they will be at Galorndon Core in six hours - then within the same scene tell Picard that they will be on the Federation side of the RNZ in five hours. If we assume Commander Tomalak isn't lying in either of these statements, and doesn't alter speed (he wouldn't be motivated to use less than maximum speed before talking with Picard, and after talking to Picard he's in greater hurry than ever, to stop his captured man from talking), we know it takes a Warbird five hours to cross the RNZ. Five hours at very high warp, then... It must be more than a lightyear or even a parsec, because we get consistent mentions of travel faster than five lightyears per day in the TNG era. It could
well be 20.00 lightyears, or a full sector side, in that case - and could have been that all along, even back in "BoT". But it would make the Warbird slower than Kirk's ship...