Quote Originally Posted by Redrobes View Post
joćo paulo has pointed out that the largest island where a river has forked around it is a mere 20 square kilometers.
http://www.cartographersguild.com/sh...28&postcount=3
This river is going through its very temporary meanderings to find one course through this region. One day soon one side will dry up. In any case 20 square km is very small.
The link that JP gives in his post actually says the island is 19,000 square kilometers (~12,000 square miles).

The Bananal Island covers an area of 19,162.25 km², twice the size of Lebanon or Jamaica. It is the largest fluvial island in the world being 350 km long and 55 km wide.
From http://en.wikipedia.org/wiki/Bananal_Island

That seems pretty large to me, but, granted, that is a rarity. The largest one I know of is about a square mile and most are a few feet.