..looks like a 22 inch paella pan?
The best/easy might be to create the pan using revolve feature and then using flat surface option (you need Premium version or check this post https://forum.solidworks.com/message/899670#comment-899670, reply #21 by Eric) to get the required blank.
You can't really work out the blank.
Spinning need a die to spun the metal sheet on to.
Also you start with a thicker material then the finish product.
Spinning actually is a cold forming process. Similar to stretching over a die.
Manufacturer need to test the first piece on the die to know what size blank they need.
So, since they'll need a die and most likely you don't want to pay for one, you'll end use whatever die they have.
Draw and dimension what you need and like the manufacturer see what they have closest to it.