What about 1x4?

And seriously, wtf are clowns? HFS?

Gotta be 4x4, I think. You're guaranteed to have at least one opening in every 4x4 area of the map. Other shapes won't ensure it (though obviously anything that includes a 4x4 area will.) A 3x3 area gives you, I think, about a 1/3 chance.

No. Word of Toady, and supported by all the careful research I've seen so far, is that there is one blue spire generated in every 2x2 embark grid, randomly placed amongst the four embark squares. Every 16x16 region square therefore has exactly 64 blue (and one special HFS). A 2x2 embark aligned to the grid will have exactly one blue spire. A 2x2 embark offset may have as few as none or as many as four blue spires. Over all the possible configurations of 2x2 embarks, your odds are as follows (rounded): zero blue, 20.4%; one blue, 60.6%, two blue, 17.8%, three blue, 1.8%, four blue, 0.1%, with an average (not surprisingly) of one blue; you also have a chance of special HFS of about 1.6%.

A 3x3 is guaranteed (barring serious program bugs) to have at least one blue, has an average of 2.25 blue, and a max of four blue, with a special HFS chance of about 3.5%. An unaligned 4x4 still has a minimum of one blue, but an average of four blue, and a max of nine blue; with special HFS chance of about 6.3%; aligning it with the grid gives you exactly four blue. Moving up to a 5x5 increases the minimum blue to four, with an average of 6.25 and a max still of nine; the special HFS chance is 9.8%. If you're planning one making the HFS a major part of your fortress and don't cheat with reveal, an aligned 4x4 is the smallest you can generate that is guaranteed to have multiple blue (and therefore a high chance of multiple entrances, reducing the risk that one of the spires will not be traversable).

Odd sizes are not recommended, they may still have various bugs associated with them. In any case, your minimum blue is controlled by the *smallest* dimension; a 1xN / Nx1 or unaligned 2xN / Nx2 can have as little as zero blue. To guarantee at least one blue, you need the smallest dimension to either be 2 and aligned with the grid, or 3.