Bob
Terminology time ... it's a mess and it was then.I can't speak for the flags on the Burgundian helmets but the pennon was a farily widely used flag throughout Europe, typically associated with units of roughly 20 men and carried as per a standard etc.
As for construction ... From most of the references to both European and English pennons they are around the 1 to 2 ells (for Flemish approx 21"-42") which would actually bring several of the 'standards' listed by Deuchler in Die Burgunderbutte into this category (although he didn't list them as such)
Looking at Richard III's Coronation accounts there is sufficient cloth to make 470 (from memory) pennons ... these are almost all Linen and would probably (based on the guestimated cloth surface area and a 3:1 proportion) be about 18" long.
However the material could be silk ... or even worsted (at least one English Standard was!)
They are seriously cheap and nasty items and almost certainly painted. Cennini gives instructions on how to do this, and varnish it. However when completed are very effective (I think the White Company web pages still have pics of the two we made (personally I'd make them slightly smaller now but the proportions are within period bounds)).
As to the construction it should be a right angled triangle. This would fit with the 'Standard of Ghent' which is a painted 'Standard' of the city of Ghent made in the 1470's and still surviving. This is Linen (If I recall correctly) with a silk fringe (I don't suggest a silk fringe on a pennon ... indeed any edging at all). It is made from two widths of linen with the join running down the length of the Standard. From memory it has a lion, the Maid of Ghent and the letter G painted on it.
I'll see if I can dig out the article (well actually an open letter)I wrote which was in he White Company Newsletter a year or so ago as that may help.
Cheers
Dave