Hey!
First off I don't think the size of the island is a problem, since floating islands and everything in the picture overall is kind of unrealistic =)
I did something similar just a while ago.
I figured you could use it as a reference when you'r making the island grass and rocks. Obviously it's a bit smaller than your picture but hopefully it can help some!
Let's get on to your picture.
I think the trunk looks really good and interesting. The foliage looks a bit boring compared. Also the grass is a bit too high saturated and you can ditch a lot of colours and get more solid patches of light and shadow. The sunchair's AA is a bit weird and I think it would help you a lot of you just had a straight line of pixels (1,1,1,1,1) intead of the 1,2,1,2,1,2.
Good luck!
/coffee