The Small 14k rose gold heart locket pendant necklace, from Apples of Gold Jewelry, measures 10mm in size (just over 3/8") and holds 2 photos inside. Overall dimensions with hoop/bale is just over 1/2" tall.
The rose gold heart locket can be engraved with one letter on each side (optional).
Add a matching 14k rose gold 0.7mm box chain in either 18" or 20" to match.
"For where your treasure is, there will your heart be also" (Matthew 6:21).