If I was to purchase a bashring to replace the big chainring e.g. E Thirteen Supercharger, what size do I get? Is it just a little bigger than my middle ring?

Thanks in advance

The one marked the same size as your middle ring will protect it. But if you go a little bigger it gives you options later on.

i got a fsa one from crc that does 32-36, worked fine
EDIT this one
http://www.chainreactioncycles.com/BasketAdd.aspx?AddToBasket=true&StockID=121378
it says 36tooth but it worked fine with a 32 til i got a 36 for some lower gears, comes with longer fitting bolts as well