The RMS value of a periodic signal is the DC (straight line) value which would transfer the same power per period. The Nyquist-Shannon sampling theorem states that in order to properly resolve a sampled signal, the sampling rate must be at least twice the frequency of the highest frequency component you want to measure.
For a pure sine wave at 10 kHz, for example, you would need to sample at 20 kHz or greater in order to properly distinguish the waveform from a similar waveform of lower frequency (see this figure). Since the period is the reciprocal of the frequency, you should select a sampling time and number of samples such that their product is less than or equal to 1/(2*f), where f is the frequency of your generated sine wave; in other words:
2f <= 1/(Ns*Ts)
A description of the block, including these parameters, is available in the PLECS manual, page 354, and can also be seen by right-clicking on the RMS block and selecting "Help".