ISSUE 1: What should the dimensions be? It must be perfectly square for easy rotation & repetition. I've experimented with 15x15, 20x20, 30x30, and 40x40 sizes. Currently tending towards 20x20 due to its not being too small or too big, and it's evenly divisible by 10 which makes Shift+<movement key> work well in DF. 30x30 gives notably more freedom in how blueprints are laid out, but may be too large for some uses -- and the whole idea of the "Modular" set is that you repeat smaller modular blueprints as often as needed.
This has a lot to do with personal preferences. Some players like to build vertically, so they'd prefer something like 10x10 or 15x15. But some players want room to expand horizontally or prefer to see a lot on the same level instead of changing levels constantly. They might prefer something like 20x20 or 30x30. Personally, I feel that 15x15 is too small, while I find 20x20 or 30x30 more appropriate. However, I don't always play the same and sometimes I go for small layouts.
That said, there are a number players (including myself) who feel strongly that most hallways should be
at least 3 tiles wide. If they're anything less, then any areas with traffic will cause dwarves to run into each other and re-path constantly, causing a significant drop in FPS. (The only exception being low traffic areas, such as seldom-used corridors to isolated bedrooms.) But 3-tile wide corridors naturally suggests a cross pattern (North-South and East-West corridors) and that results in an
odd number of tiles for dimensions. So instead of 20x20 or 30x30, it'd be more like 19x19 or 29x29. Some of us
absolutely refuse to consider blueprints with corridors 1 or 2 tiles wide.
I'm just saying there are a number of players who are
much more concerned about maintaining good FPS and having their dwarves finish hauling jobs and get to their destinations faster than worry about making Shift+<movement key> work well. Consider that FPS death is
still a leading cause of fortress failure (for older, established forts) and that each new release of DF is larger and with more features (theoretically making FPS worse). Many of us cannot afford to buy a new computer every other year and ToadyOne is not likely to address these FPS issues for a long time yet, either.
For reference, here is how many bedrooms and the bedroom densities I was able to obtain at different blueprint sizes, using 4 cells per bedroom:
dimx dimy rooms area rooms/sqft rooms/edgelen
20 20 28 400 0.07 1.4
15 15 16 225 0.071111111 1.066666667
20 30 39 600 0.065 1.95
30 30 64 900 0.071111111 2.133333333
I think a lot of players still use the old 1x3 style (3 cell) bedrooms for peasants, reserving anything larger for nobles and the like. (Or would that make 4 cells with a door?) But, again, this has to do with tastes, preferences and play style.
BTW: I see nothing wrong with having, say, 30x30 or 40x40 size sleeping levels and smaller levels (such as 20x20) for all other levels... Well, except that any stairs near the corners would not line up with other floors, making a large central stairway important.
ISSUE 2: How should the stairs be arranged? Three basic possibilities:
a - 2x2 staircase in the center of the blueprint
b - 1x1 staircase in each corner of the blueprint
c - 1x1 staircase at the midpoint of each edge of the blueprint
I'm tending towards option b currently, because it leads naturally to a template design where the outer edges of every blueprint are 1-wide hallways with stairs at each corner-intersection. When such a blueprint is repeated 2e, the 1-wide hallways on each blueprint combine to form a 2-wide hallway between them. Repeating 2e 2s, we get a 2x2 staircase in the center. This helps the fortress hallways/stairways scale up the main thoroughfares as the fort increases in size.
If 20x20 or larger size levels were considered, then having at least 1 staircase near the corners seems the way to go, rather than a large central staircase. However, if the level is large enough, I could see having both a central staircase and some staircases near the corners.
Anyway, it sounds like you did not even consider 3x3 staircases (to go with 3-tile wide corridors). I've looked at a lot of fortress maps, DF YouTube videos, and fort designs and a surprising number of them use a 3x3 central staircase. At least with recent DF versions, they seem more common than a 2x2 central staircase design.
...The main downside of this option is what to do when you get to the surface level. Now you've got 4 1x1 staircases emerging at the surface. This makes surface blueprint design rather tricky and/or you try to funnel those 4 1x1 staircases into a centralized 2x2+ staircase on the first sub-surface z level. Either way seems like a bit of a hack and may stymie efficient dwarf movement from surface to underground.
Yes, having 1x1 staircases in each corner would cause a problem for the z-1 level. Most players only want 1 main entrance to their fort for security reasons, at least to begin with. (However, 2 or more entrances are doable later, esp. for experienced players.) As you said, one solution would be to have the z-1 level with 1x1
down staircases in the corners and only have a centralized staircase (or side entrance) to breach the surface.
As you said, the problem would be funneling that traffic through a 2x2 stairway to the surface. But this is less likely to be an issue with a 3x3 central staircase going all the way from the surface to the bottom. Even if it has 1x1 staircases in the corners, the 3x3 central staircase should be able to handle the traffic volume to and from the surface.
Another solution might be to have a completely enclosed (with roof) surface level with one or more bridges as the entrance(s). Depending on the design, the corner staircases could be functional as there might be a need to go to the roof, such as for marksdwarves to reach the fortifications in order to patrol them.
(BTW: Personally, I like the idea of using the central tile of a 3x3 staircase to build an automated waterfall system to keep dwarves happy.)
Edit: Oh, and another reason why I prefer odd-numbered dimensions (such as 19x19 or 29x29) in blueprints is it makes it easy to center it. For example, I like to use something like:
#dig start(4; 4; Center tile of a 7-tile square)