You can change the ASIO buffer size within the DAW set up pages. You cannot change the Firewire buffer size when the DAW is running. This can be done by going to the Windows Start menu > All Programs > Focusrite Saffire Pro > ASIO control panel.
The firewire buffer size determines the step size of the ASIO buffer choices. For example a firewire buffer of 4ms means that the ASIO buffer can be set to 8, 12, 16, etc ms.
Older, slower computers will require larger firewire buffers sizes to prevent break up in the audio playback.