I bought two initials and "&" which I found at Craft Warehouse, though I've seen them in other craft stores too, and online. These are about 24 inches tall.
For the "&" I cut the front layer of cardboard off with a razor blade. The letters were super easy--leave them uncut. Two coats of matte black spray paint around the fronts and all edges, and they're done.
I traced the "&" out on a foam board and cut it with my Styro Cutter. I sanded down any parts where it didn't fit just right and then filled the empty shape with the foam. To minimize any white showing through I painted the foam pink (even though foam doesn't paint great, it worked well enough.)
When the paint dryed, I cut fake flowers off their stems with wire cutters, leaving about and inch still attached to the flower and stuck them down into the foam until the shape was filled with flowers. I used larger flowers to accent the initials, and just attached them with hot glue.
Depending on where you want to display these giant cardboard letters, you could find a way to hang them--string, tabs on the back, etc. We just propped them up against the table while they sat on the ground and that worked great too.
Where would you use these letters?