root@OpenWrt:~# mount
/dev/root on / type ext4 (rw,noatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/mmcblk0p1 on /boot type vfat (rw,noatime,fmask=0022,dmask=0022,codepage=437 ,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
By default the root partition is about 250 MB, so the same image will fit on practically any card. The rest of the card is unused regardless of its size. There are two options to set up and use the rest of the space.
Create and format another partition, then mount it e.g. as /srv or /data,
Expand the existing root partition and its ext4 filesystem.
Unless you're a real CLI die-hard I'd suggest attaching the card to a Linux desktop and use gparted for either of these operations.