255/55R19 is a tire size specification:
255 = tire width in millimeters
55 = aspect ratio (sidewall height is 55% of the width)
R = radial construction
19 = wheel/rim diameter in inches
For this size:
Tire width: 255 mm (10.0 in)
Sidewall height: 140 mm (5.5 in)
Overall diameter: ~29.0 in (737 mm)
If you're looking for:
A replacement tire recommendation
Equivalent/alternate sizes
Whether this size fits a specific vehicle
tell me the vehicle's make, model, and year.