Wizards posted an article about mapping with hexes and scale. I played with hex maps a lot over the years. In particular how to join hex maps together and how to manage a hex map when making a hexcrawl setting.
Writing sandbox settings can be painstaking work at time. Even writing tersely the quantity of locales can be overwhelming. My observation that you will have a dozen or so really good ideas and the rest you crib from whatever idea generator or random table you have.
You could limit this but you don't want to make the region so small that the player are able to move past it's bounds in a session or two. Nor you want to make the locale spaced so far apart that that you get the howling wilderness of the 30 mile hex.
I found that making your hexes between 3 to 6 miles to be ideal. If you are using 1/2" hexes on a 8.5 by 11 paper you get a region of 135 miles by 90 miles. It comes roughly to 27 hex columns and 19 hex rows. For that size three dozen locales fit nicely.
For a 22" by 17" map, the size of a Judges Guild Wilderlands map, detailing that many locales becomes a bit of chore. If that what you want to do find some good random tables to help as an idea generator.
So you start off your campaign a letter size paper full of 1/2" hexes. Then you decide to expand the campaign. How to do you make sure everything lines. The easiest method to have some overlap, generally a
hex column, or hex row is sufficient to keep everything consistent across multiple maps.
There are two types of hex grids
Of the two, the vertical hex map is by far the most popular.
The examples in this post will be using the vertical hex grid.
Hexgrids have several choices how they can be formed.
You can make the end columns even in number.
You can make the ends uneven in length.
The last arrangement is used when you sub dived a large hex into smaller hexes.
If your map just going to be one page. Then you don' t need to worry about how to join two hex maps together. However with multiple maps then this issue needs some attention
For vertical hex grids the top and bottom of the map are uneven. If you don't want to overlap then you have to have one page start the first hex column high and the lower (or upper) page start the hex column low.
But there are other ways of handling the vertical joining of two hex grids and they have the virtue of making it easier to avoid mistaking in drawing features like rivers and coastlines across multiple maps.
A half overlap
A full overlap
Of the two I prefer the half overlap. It slightly spreads out the vertical coverage of each individual map and I only have to copy the top and bottom hex every other column.
The horizontal joining has several types.
If you use a hex grid with uneven numbers of hexes you can lay them side by side with no overlap.
Judges Guild in the Wilderlands of High Fantasy was one of the first publishers to deal with this issue. They used 18 hex maps arranged in a three maps across 6 maps vertically. Each Hex maps had 52 columns, and 34 rows on the odd columns (1,3,5, etc) and 33 rows on the even columns. The resulting hex grid had uneven ends on the left and right edges.
They decided to use the half overlap to join the maps on the top and bottom edge. However they messed up on the left and right edge and decided to make them overlap. Because of the even ends of the hex grid this resulted in a staircase effect as below. Each map to east was a half row south of the map to the west.
For hex grids that have even ends you can do a full overlap of the last hex column with the first hex column of the next map.
I prefer the full overlap option as it helps ensure that I am correctly drawing from one map to the next. The same reason applies to the half overlap option.
For vertical hex grids the numbering system is XXYY where XX is the column number and YY is the row number. This is reversed for horizontal hex grids.
Judges Guild is famed for having a complete mapping system that goes from campaign level of 5 miles per hex to a regional level of .2 miles per hex and finally to a local level of 42.24 feet per hex. Each larger hex was subdivided by smaller hexes 1/25 th the size of the larger hex. Hence the odd number at the local level.
If your scale per hex is an odd number (5 miles, 25 miles, etc) it is easy to draw up a subdivided hex as shown below. You pick a center hexes and count the remaining hexes outwards. You can use the six points to draw up the six sides of the larger hex.
Hexes with a even scale (10 miles, 30 miles, etc) are not as easy to subdivide. The lines you will be drawing for the sides will be meeting in the center of hexes.
There is alternative for drawing even scale hexes but you will lose the center hex. You will have decide which form is best to use for you game.
I hope with the Wizards article you find this results for your mapping with hexes.