It would be interesting to know how people use the various AF options. I would be interested to know if the AF point on the 1D4 are the same size as the 7D or whether, as I suspect, they are smaller.
They are the same size, which makes perfect sense from an optics standpoint. Canon uses the same 48-bit lines for their sensors (sometimes in offset pairs they call 'dual zig zag'), and an f/2.8 baseline is the same physical size regardless of sensor format. If you take an image of a 1D IV AF sensor and superimpose it on an image of the 7D sensor, you can rotate one of them 45 degrees and see that the spacing of the f/2.8 lines is the same for both. That's physics.
The exception is the center point on some 1-series bodies, where 96-bit lines are used (same length, twice the density, which is how they manage accuracy equivalent to Canon's normal f/2.8 and f/5.6 at f/4 and f/8, respectively).
Sidebar: that 'trick' may be exactly how Nikon is offering many f/8 points on the D4 - they have the lines spaced at f/8 spacing, but gave them double density to make up for the shorter baseline.
