Sampling Rate:  Is essentially the precision in which a digital file represents the original sound. The sampling rate is the rate of samples captured per second which is measured in Hertz (Hz). Generally speaking, the higher the sampling rate the closer the actual recording will be to the original sound.

When recording sound, the sampling rate should be at least twice the highest frequency for good audio quality.  For example humans can not hear frequencies above 20,000 Hz, which is why a music CD has a sample rate of 44,100 Hz, or 44 KHz for short. A sampling rate of 44.1 KHz provides excellent clarity for encoding frequencies up to 20 KHz.   

For best speech recordings, it may not be necessary to have the same clarity as a music cd.

Sample size: Is essentially the range of values that can be assigned to each sample, higher sample sizes provide more dynamic range (meaning lower lows and higher highs in sound).  More simply put, the greater the audio sample size, the better the quality of the digital audio. Sample Size for audio is most often expressed as 8 bits or 16 bits. CD-Audio discs are rated at 16 bits. However 8bit audio is more often used for voice recording.