Using two (1, 2) reference pictures I discovered, I came across the achingly familiar problem with cosplay prop design: no two stills are ever alike. The proportions change, the shape changes, the design changes, the color changes, everything changes depending on the creator's flight of fancy and level of detail needed for the scene. So as usual, it came down to eyeballing things. In this case, I realized that the daggers must be slightly longer than the character's arm from inner elbow to his fingertips. Having gotten that length from my client's arm, I had something to work with.
The basic proportions, have some almost painful work, ended up being roughly that the blade was 3/4 again as long as the hilt+guard. So Hilt+Guard = 7.5in, Blade = 12.5 for a 20 inch dagger. The proportions were not fun to do and took me a few hours to manage. Oh who am I kidding, I love figuring out the math. What I do not like doing is trying to measure things from an image on my screen. I also went about measuring the broadsword I have in my closet to find out the width of a comfortable hilt as well as the proportion of the blade to the hilt. You can tell from the pictures that the hilt is thinner than the blades, but thicker than the gap between them, and the guard is wider than the blades. I am pleased with the way the design ended up coming out.
Next up is cutting out the design from the cardboard (got client approval for the design late last night) and tracing it out onto foam. Then begins the part of prop design that always ends with aching hands and sore wrists... cutting and sanding.