All measurements are based on diagonal screen length.
Small: under 4.5” screen (11.4cm)
Medium: 4.5”-5.4” screen (11.4cm - 13.7cm)
Large: 5.5”+ screen (14cm+)
As a reference, an iPhone 7 is considered a medium size phone.
You can use our phone finder to help you select the right size phone for you.