Linux

$ fallocate -l 10G gentoo_root.img

$ dd if=/dev/zero of=./gentoo_root.img bs=4k iflag=fullblock,count_bytes count=10G

$ truncate -s 10G gentoo_root.img

OS X, Solaris, SunOS and probably other UNIXes

 # mkfile 10240m 10Gigfile

HP-UX

# prealloc 10Gigfile 10737418240