As a bare minimum a 2GiB card is needed, but at least 4GB is recommended. Note that you need a card that is at least 2012MiB in size for the image to fit. Many cards advertised as 2GB are actually smaller than 2GiB, so to be on the safe side, get a 4GB card, otherwise you will most likely find the space constraint uncomfortable to work with. If performance is a concern, choose the model of SD card carefully. The only ones found in testing to have reasonable performance (i.e. at least comparable to mechanical disks under a random-write workload) are "SanDisk Extreme Pro SDHC UHS-1 95MB/s" models. See here for more details (link at the end of the article, then scroll down to the second table):
Get it from here: http://ftp.redsleeve.org/pub/el6/rootfs/rsel6-cuboxi-latest.img.xz
I used my Chromebook to prepare the SD card. The external SD card appears as /dev/mmcblk1 on those, to the instructions here reflect that. Adjust the device node name accordingly for your system.
# xz -cd rsel6-cuboxi-20141025.img.xz > /dev/mmcblk1; sync
Put the micro SD card into your Cubox-i and power it up. The default root password is "password". You will probably want to extend the partition and file system to fill your SD card. Make sure that you make the re-created partition begin at 2048 sectors. Then use resize2fs. From there you can configure the ethernet port and use yum to install any other packages you require.