It's about 2.5-3mm. I left the sides uncovered as the wire ends around the edges appear to be good nucleation sites.
My device is as basic as it comes straight DC no polarity switching (when I cleaned it I switched the polarity at the connection to the power pack so both meshes receive the same wear over time), from what I have read Pulse Electrocoagulation is the most effective method where the voltage is applied pulsed in the KHz range but it also has the polarity reversed on each pulse, which as Ian describes above kind of ACish, so a circuit needs to be designed to achieve this for DC, something flip flop something something (not found a good circuit design yet, a dedicated pulsed power supply for electrolysis would be good to have but they are pretty expensive).
There's a lot more going on with the polarity reversal and the high switching speed than just electrode passivation/dissolution mitigation, from what I have read in a paper (can't remember which one) it appears that the effect of Hydrogen and Oxygen being formed next to each other on the same electrode (polarity reversal) only separated by a very short time (pulse interval) that at the nano scale the gasses are cramming themselves into the interstitial spaces between water molecules and if in close proximity to each other they can solve back into water, which if I remember rightly was what Twinstar at one point said their device made as a byproduct, which prompted scepticism, well apparently the effect is real.
With the above in mind and my DIY effort not being pulsed polarity reversed I kept the electrode distance small so as to increase the chance of this happening spontaneously.
Regarding electrode substrate type, Twinstar is very likely to be using Platinized Titanium mesh for the electrodes, the reasoning for Platinum is that it is much more resistant to passivation and dissolution than other metals so the electrode longevity is more prolonged, problem is it's expensive so the cheaper solution is to coat another cheaper metal with it preferably one that has low reactivity like Titanium. The problem with these types of electrodes is that if dissolution occurs and the platinum coating is breached and the titanium then exposed it will form an oxide layer beneath the platinum causing it flake off (this has been seen on the Twinstar electrodes when they have gone kaput).