According to Tyres.co.uk, the 2003-2005 CLK 240 Cabriolet comes equipped with Michelin Primacy 4+ and Goodyear EfficientGrip Performance 2 (fronts: 205/55 R16 91V and rears: 225/50 R16 92Y). Confirm vehicle registration as wheel options and fitment vary.
The size of the Mercedes CLK tyres can be found on the tyre pressure sticker in the fuel filler cap on the right-hand rear of the car. The sticker has all approved dimensions and tyre pressures. Also check the sidewall of the tyres and the handbook as tyres fitments and generations of the Mercedes CLK wheel options can vary and there can be front and rear tyres that are staggered.
It is not necessary to replace all four tyres, as factory Mercedes CLKs are rear-wheel drive, so they do not have a drivetrain-based four tyre matching rule. As CLKs have staggered wheels it is imperative to maintain the factory front and rear tyre fitment. If you are replacing two tyres on the same axle, as per UK safety recommendations, the two should be placed on the rear axle.