[knit4ES]

How big are you thinking of? 12 inch? Usual sites, meaning Ravelry and Googling "knitted turtle"

ETA --- Here's a list of some... they are all about 6" around, but it seems to me that they could be adapted to be bigger....
Sheldon Turtle - An adorable turtle that is 6 by 9 inches with great style, available at http://knitty.com/ISSUEwinter06/PATTsheldon.html

Tuck The Turtle - A pleasant little turtle that's easy and cute, found at http://www.spudandchloe.com/blog/2010/03/tuck-free-pattern/

Sock Turtle - An easy pattern that is very light and happy, found at http://mybooteeknits.blogspot.com/2008/05/introducing-newest-member-of-mybootee.html

Dylan's Turtle - A simple design that ends in a wonderful turtle creation, found at http://knittybutton.com/2010/04/11/a-turtle/

 

[knit4ES]

knit4es the turtle cushions were probably large enough for a small child to sit on. the shell was probably 19" across.

Ahh... I'll keep looking.... may be time for a pattern to be designed... starting with an oval and then adding the head, legs and tail.
One of those listed above had a slip-stitch pattern for the "shell" on top. Seems like I've seen something similar in a honeycomb stitch too.

 

[knit4ES]

Okay, I was searching for some other things.... here's a turtle shell that might help you get started with a design
http://lahelaknits.co.uk/patterns/mariokartshellredorgreenpattern.pdf