Thank you very much Noraiman, I thought it is splitting video by duration instead of picture, I never thought that a few 1080p frames would take over 16GB VRAM. Looks DAIN has a long way to go.
I've tried many different size values and I think 900px may be the limit for 16gb, very close to ur calculation. If it goes higher rendering process will be unstable, some scenes may take more memory.....Or maybe just my video bit rate is too high, each frame after extraction is about 3MB, I haven't try other video so not sure if this size is normal.
I temporarily gived up proccessing this video, for it will take about 860h to rendering 🌚
About different CPUs, I noticed CPU usage is low after frames extraction, so yes cpu is probably have no thing to do with rendering speed.