Medium size most common 29mm x 15mm x 7mm.

Will fit:

Singer model- 15-86, 15-88, 15-90, 15-91, 66, 99, 185, 192, 201, 237, 239, 242, 247, 252, 257, 301, 327, 328, 338, 348, 360, 401, 403, 404, 413, 416, 457, 500, 503, 513, 533, 534, 543