The maximum spark gap on the arc generator specified in this project, and all comparable generators for that matter, is 10 - 20 mm. Using this value, electrodes set up in a grid pattern (two rows, multiple columns) need to have the maximum distance between electrodes (horizontal or vertical, diagonal if desired) can be 20 mm. I will probably start with 15 mm spacing between electrodes as specs are often 'generous'.
For addressing, we should probably start with fixed polarity pairs, eventually moving to single electrodes with switchable polarity. These fixed polarity pairs should be the max spec distance plus margin away from other pairs, so about 25 - 30 mm will do. We should end up with no more than 10 pairs, and this many would only fit on the 'large' size. For a one size fits all, we should be looking more in the range of 8. This seems manageable.