Natroy there is no specific size or shape it needs to satify the rules as the requirements of each bike is very different.
My YZ250L required a small piece of plastic sheet (old number plate) added to the rear of the rear chain guide to make sure it complied which was about 50mm x 40mm. My 77, 80 & 81 Montesa Cappras required a small triangle of aluminium 50mm x 50mm x 50mm added to the rear chain guide. My VR250 needed a much larger guard made to cover from the brake stay mount back to the sprocket (about 75mm by 125mm aluminium).
The rule states
16.15.4.6 A chain guard made of suitable material must be fitted in a way to prevent trapping between the lower drive chain run and the final drive sprocket at the rear wheel.
People seem to be very put out about this but is seems logical to me that the rear sprocket is just as likely to cause injury as the front sprocket. Yes we have raced for years with no problems but it is not really a big issue. It is like a lot of safety items it is only when your finger or someone elses is stuck in the chain that you think a guard would have been so much easier 
The mission statement of the Politicaly correct movement 
So at what point do we start fitting covers over those nasty spokes , shit theres 72 of the dangerous buggers spinning at speed on a jap bike and 80 on the Pom bombs and some euros just waiting for somebodys hand to be lured in
And surely all those nasty hot exhaust should be wrapped in 3 foot of
abspestos cloth something heat proof but PC , just in case
Any sane person would replace those ridiculous nasty handle bars and dreadfull lever things with a nice padded steering wheel , paddle controls and air bags just in case
Who in their right minds would risk life and limb by putting their feet on those medievial looking sharpened devices
Surley a nice set of big running boards would be so much more in keeping with the modern PC way , you know JUST IN CASE
