It all depends on your computer. in JRiver, using ASIO, the buffer size is recommended at 500 milliseconds. In the iFi driver settings, this greatly depends. Optimally, for the best sound and clocking, your USB Streaming Mode will be set to: Minimum Latency. The only way to tell if your machine can handle it, is to listen. If you hear no clicks, pops, skips in the songs, then you're probably good at Minimum Latency. But also consider the ASIO buffer size, which recommended at 8192 samples.
You will only know by trying. Start off with 2048 samples and "Safe" USB Streaming Mode. If you could none of the aforementioned problems, then kick it up a notch to 4096 samples and "Reliable" in the USB Streaming Mode. While you're doing this, you can always check your CPU usage, by going to your task manager and check. You can even go to the resource monitor and double check. In other words, if you're trying to get to Minimum Latency and 8192 samples, you can see how much CPU and memory your machine is using to process all of this data.
For instance, me, I might peak out at 23% CPU usage, while really putting my computer through the paces with DSD files. That's not too bad. I usually have about 2GB of memory in reserve, too. Often before listening I'll do a quick restart, just to make sure my machine is running the smoothest it can.
Even at that, though, if I'm doing anything else on my computer while taxing it a bit, I'll get a little stutter in the audio, which tells to stop screwing around and just listen.
So, basically it's trial and error. You'll get there.
Oh, and in your options for Bitstreaming, set it to DSD. You shouldn't have to upsample anything from there, unless you really feel the need to (it's debatable if you'll hear the difference). If you don't want to jack with upstreaming, make sure in your DSP output format dialog box, that you're Output Encoding is set to: None. Your channels are set to: Source Number of Channels and your Sample rates are set to "No Change" all the way down the row.
