Please, you must be print this sheet. It contains the EXACT MEASUREMENTS but only printing it in DIN A-4 CONFIGURATION (printer default configuration).
After that you can look for your heel tips putting your shoes or boots over the sheet. See the photo:
If you have not printer:
You can measure the wide x long of your old heel tips. Or better if you take out them, you can measure the wide x long of the heels of your boot or shoe.
 |  |
| wide x long (axb) in mm | For example: 17x18 |
All Our Measures:
Heel tips U shape (wide x long in mm) : 8x8, 9x9, 10x10, 11x11, 12x12, 12x15, 13x13, 14x14, 15x15, 16x16, 17x17, 18x21 (18L), 18x24 (18SL), 20x20
Heel tips round shape (diameter in mm): 8, 9, 11, 12, 15
Heel tips square shape (wide x long in mm): 18x34