KK usually just says "sleeping lion," given that not all of her readers can be counted upon to know heraldic terms, but the proper heraldic term for that posture would be "dormant." And your second lion in that post looks extremely familiar to me; that's the one my DH uses on his SCA fighting shield and his painted banner!
IIRC it's line art from The Art of Heraldry
by Fox-Davies, though I could be mistaken on DH's source. He had his SCA device long before we met. DH's lion is "couchant" rather than "dormant," so that third lion in your post would be more accurate for Duncan's device.
One problem is that I don't know what color that lion is. Is it gold? Red like the roses? Is it "proper" (i.e. colored like a real line, with some leeway for the artist because real lines come in various shades of color, but one can assume a large tawny cat with either some similar colored or black mane)? Unless KK has hidden that detail somewhere in the novels and I'm not remembering where it is, it's going to be hard to create a banner for Duncan. Maybe she'll be at Sunday chat and can answer that question for us.
My guess is that Duncan's escutcheon is similar to your top one, only with the shield being blue at the bottom and white or silver at the top, the roses being red as in your sample, and the lion being some unknown color at this point. But since I don't know if those field divisions are accurate, that's just a wild guess. It could be that the lion is in a blue field within a white border that is sprinkled with roses. Or that the shield is divided vertically or diagonally, etc. We know from HD that the Cassani livery is sky blue and silver (and given how expensive, not to mention impractical, genuine cloth of silver would have been to purchase for an entire army, not to mention keep untarnished in battle conditions, I'll assume KK means "pale light grey"), so it's possible that Jared's shield device reflected those colors as well, although that would have been due to artistic preference, since there wasn't a heraldic differentiation between "blue" and "sky blue" in the Middle Ages yet (unless of course "bleu-celeste" as a heraldic color came into vogue centuries earlier in the Eleven Kingdoms, but that would be up to the Author), and "argent", while literally translating to "silver," is quite often painted in as white when the heraldry is emblazoned on something like a shield or banner.
So as for size.... Gee, I dunno. 4" x 6", maybe? That's about the same ratio as my baronial banner, but scaled down to a reasonable size for an action figure. And if an extra bit of fabric is left at the top (the flat edge), it could be folded over to create a casing for a wooden dowel that would allow for something like this:
That's a versatile "hanger" for a banner that allows it to be hung up on a wall, or suspended from a pole with a hook, or any number of other means of display. My SCA banner used to hang with DH's from cup hooks in the wooden beams of our Great Hall/living room, on either side of the fireplace. DH also uses them to mark which pavilion is ours when we're at an SCA camping event. Lots of handy uses for a good banner!