Essentially, the maximum distance between the electrodes is a function of the ohmic drop across that distance and, therefore, a function of the conductivity of your experimental solution.
That said, apart from the possibility of the electrodes actually touching each other (or the deposited layer forming a connection betwen them), I see few reasons not to employ the smallest feasible distances between the electrodes.
Essentially, the maximum distance between the electrodes is a function of the ohmic drop across that distance and, therefore, a function of the conductivity of your experimental solution.
That said, apart from the possibility of the electrodes actually touching each other (or the deposited layer forming a connection betwen them), I see few reasons not to employ the smallest feasible distances between the electrodes.