I have nuendo and I record my project in my m-audio fw1814.
In nuendo I use the asio m-audio drivers,but sincerly I don't know what is the best buffer size to set in nuendo and in my audio card....actually I have the bufer size at 256.....
so if you don't monitore directly (for ex when you're using guitarrig or similar and need to hear the processed signal) set it low (3-6ms) during mixing use higher latency (buffersize) for less cpu-usage
dependas on how you monitor. in case you monitor post nuendo you need a low buffersize, if you monitor the "direct monitoring" output of you interface and leave the little speaker-symbols in nuendo off latency doesn't matter