What framesize is usually used for audio effects?
Typically this requires a trade off between CPU efficiency and latency. Frame based signal processing incurs at least one frame of latency and it you are planning to play live through the effect, you need to keep this quite low otherwise it get's distracting.
On the other hand, frame-based processing is way more efficient than sample by sample processing, since you can use DMA, vector instructions, highly optimized loops etc and you can amortize setup (coefficient calculation and loading, loading and storing of states, etc.) over the entire frame.
For live audio effects which moderate algorithmic complexity, something like 32 or 64 samples at 44.1 kHz or 48 kHz typically is a good starting point.