This is mostly from my gut feeling and reasoning, but I present an opposing view.? I would tend toward a wider gap rather than narrower.? But there may be a lot of leeway.
The ends make a small capacitor, which limits the minimum total capacitance and the maximum frequency to which it can be tuned.? If you make the ends very close together, you increase that capacitance and lower the max. frequency and the total tunable frequency range.
For a transmitting antenna, the ends have VERY high voltage so there is a voltage gradient across that gap.? Too close, and it will arc.
25mm is an inch.? It might have come from an American who used an inch as a round number, which got translated to Metric.? What's the breakdown voltage of air?? And how large a peak voltage do you have there?
Performance-wise as an antenna, I suspect the gap doesn't make much difference.? The high current in the loop follows around the loop and into the wires or straps that tap off to go to the tuning capacitor.? Any loop material beyond those points doesn't carry the loop's high current; they just make a capacitor, in parallel with your tuning capacitor.? They probably do not radiate anything significant.
