It's done! The polymer clay heart measures about 2 and 1/4 inches wide (5.7 cm) by 2 inches high (5 cm). The thread used was Flora size 10.
Detail of note: the ring just before the large round bead is where I began tatting the neck cord, using a Loop Tatted Ring (LTR) to get both threads through the bead without having to unwind any shuttles. This is the technique used by Bina in her Candlelight Snowflake. The LTR is a new trick for me - I don't quite have it mastered yet, but it came in handy for this project!
The closure is a toggle that I made of polymer clay, and can be inserted into any of the tatted openings for an adjustable necklace. I'm very happy with how the necklace turned out.