buffer size with realtime fx

Are you tracking? That all depends on your system and interface. Best bet is to just do some experiments with the Performance Module visible in the control bar. Set up the project with all the stuff you intend to record with (although it's better to use a few effects as possible while tracking), start record some tests and keep an eye on what the Performance Module is showing. If it's maxing out the CPU or RAM or getting close raise the buffers a little and try again. Once you figure that out then do a more audio based test. Record something for a while and check the track for pops, crackles, dropouts, etc... If there are none and the Performance module is in a reasonable range that should be good enough. The more effects/tracks you add the more taxing it will be on the system increasing the chance of problems so freeze any effects/synths and if absolutely necessary archive any tracks you don't need for tracking. All this MAY induce latency on an underpowered system or if your interface isn't very good. If you can't get the right balance between low latency and lack of audio glitches you need to look at optimizing the system or upgrading the CPU/Interface. A call to tech support should help you figure out whether that is necessary.
 
If you mean simply for editing and mixing... then it doesn't matter how high you set your buffers. The higher the buffers the less your system has to work and the less problems you will experience. It won't interfere with mixing editing. The worst that'll happen is you have to wait for a VERY brief moment when you start playback and will likely not even notice. You only really need to worry about buffer levels when tracking as far as I know.
 
Cheers.

Yeah it really is a system by system type of thing. The Performance Module as I said should give you a good idea if you are pushing your system too hard. You can add that to the Control Bar by right clicking the Control Bar and selecting "Performance Module" until it appears (you may have to do this a couple times until it replaces one of the other modules).
 
If however you are merely recording some spoken word without any backing music then you could get away with turning up your buffers a little more because it doesn't have to follow a meter. It just might sound a little echo-y while you do it. You could also crank your buffers right up and just not listen to the input monitoring at all. If you are just talking into the mic I don't see why you'd even really need to hear it and in fact for something like a sermon it might be more natural WITHOUT hearing yourself. Just keep an eye on your levels to make sure it's not clipping and let 'er rip.
 
Cheers.

thanks brother you are very helpful