Ooh, my mistake. I didn't use calculator. I thought I "remember" it, and then I spent a lot time to tried many firmwares, lol. It's was 64M out of box.
RMerlin, thank you for making things clearer to me.
The thing confused me was when I had RT-AC68U A1 and TM-AC1900, before and after flashing 3.0.0.4.376.3626, I checked rootfs size with command 'cat /proc/mtd', rootfs changed from 32MB to 64MB(from 03xxxxxx to 06xxxxxx). With this RT-AC68P B1...