[defanor]

A Festival's English model, festvox-kallpc16k, is about 6 MB, and it is a large model; festvox-kallpc8k is about 3.5 MB.

eSpeak NG's data files take about 12 MB (multi-lingual).

I guess this one may generate more natural-sounding speech, but older or lower-end computers were capable of decent speech synthesis previously as well.

[Joel_Mckay]

Custom voices could be added, but the speed was more important to some users.

$ ls -lh /usr/bin/flite

Listed as 27K last I checked.

I recall some Blind users were able to decode Gordon 8-bit dialogue at speeds most people found incomprehensible. =3

[anthk]

I'm not blind but spoken English it's far more difficult to grasp than written one (I'm a non-native speaker), and Flite runs on n270 netbooks at crazy speeds with really good enough voices.