The Classic M6 Modular Armband fits the iPhone 11 Pro 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 M6 Modular Armband for iPhone 11 Pro fits cases up to 150 x 75 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
Large 13 - 18 inches (33 - 46 cm) - Fits large arms or over clothing
Extender + 4 inches (10 cm)