Hi, I've got a 300ex which doesn't have any skidplates but that plastic one under the engine and I was wondering if anyone knew any what kind of metal should use and the best measurements.

 

Believe it or not, plastic skid plates work better than metal ones some times. Plastic is more likely to absorb an impact and slip off or over an object were metal with sometimes stick. This is one of the reasons snowmobiles have plastic skies or ski covers now.
Just my 2 cents.