$ fallocate -l <length-of-file> -n <filename>
KiB (=1024), MiB (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB, and YiB (the "iB" is optional, e.g., "K" has the same meaning as "KiB") KB (=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB, and YB.