Any ideas as to what shape this is please?

There were 6 of these along with a full set of Sapphire. 
I was lucky enough to be able to just get one for $5. 
I have had a quick look, but can't find a match.
Measurements are 157mm x 227mm x 23mm
There are no markings, but it feels right for Crown Lynn.

Is it same as the Modello Platter you got Ev?

Hi Kat, no that is like a platter size and this is not much bigger than a stretched side plate.  I might look into that though as it could lead to something.  Thanks.