Here is one of those commercial floating islands. The holes in it are approximately 8" diameter and go all the way through. There is some fabric at the bottom to hold the dirt in place. It is recommended to use dirt that is made for hydroponic gardening. The island is either 6'x8' or 8'x10'.

[Linked Image from i.imgur.com]

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).