Myoglobin (Protein Data Bank structure: 1mbn): the classic Sperm Whale myoglobin structure elucidated by John Kendrew et al. (with water bound to the heme iron rather than oxygen), and depicted in stick form, with the heme cofactor and surrounding side chains being represented in stick mode (i.e. without hydrogen atoms). This model is over 20 cm in diameter (scale: 5 mm per Ångstrom), suitable for (small) classroom demonstration. I haven't printed it out yet, so it's not yet currently for sale, but if you really want to give it a go, contact me.