I have a voice recording that must be reduced in size. It must be a wav file. how to do this? 
If it’s stereo, you could make it mono (reduces the size by half).
If it’s a high sample rate, you could reduce the sample rate.
If it’s high bit-depth, you could reduce the bit-depth.
If ADPCM encoding is acceptable, you could convert to ADPCM encoded WAV.
What’s it for?
Where did the WAV file come from?
What is the full and exact specification for the file format that you want?
What is the format now?
Its a wav file. Its a vocal only outgoing message for a phone voicemail. So quality is not that important. Mono and lo fi is fine. How do I do those things you mentioned.
If it’s stereo, use the drop-down menu to the left of the track > Split stereo to Mono.
[X] delete one of the two tracks.
Use the Project Sample Rate lower left Audacity Window and reduce the sample rate until you get what you want.
File > Export > Export WAV
At 8000, the sound is a little better than a telephone.
Koz