There are two factors that will impact how much memory is available:
Are you talking to a JavaScript program or to a iOS/Android program using the PebbleKit iOS/Android libraries?
In the case of JavaScript, you will have much more memory available because Pebble will use the same buffer that is used for installing apps and upgrading the firmware. Unfortunately the channel to communicate with third party apps written with the PebbleKit native libraries is much smaller (about 500 bytes).
The version of Pebble OS you are using
There will be small differences between versions but nothing major.