If the audio is just speech based then go for a mono 22.05kHz MP3 in 64 or 32kbps. Also, if you have the option, choose CBR over VBR. Most of the AAC files we convert are layer3 which is the same as mp3 so it wouldn’t alter the filesize massively.
As leffeboy says, it depends on what you are using to stream.
Best thing is to experiment or ask the streaming platform folk what they prefer.
good luck
TRH (sound engineer 17+ years)