The Classic MX Modular Armband fits the iPhone XR with no case or a thin case and is the best armband choice if you want the least amount of bulk on your arm while running or working out.
The Classic MX Modular iPhone XR Armband fits cases up to 162 x 83 x 10 mm.
Strap Size Arm Size (upper arm circumference)
Small 7 - 9.5 inches (20 - 25 cm) - For THIN arms or wrists only
Medium 10 - 14 inches (25 - 36 cm) - Fits most arms/most common size
Large 13 - 18 inches (33 - 46 cm) - Fits large arms or over clothing
Extender + 4 inches (10 cm)