Range cookers usually come in 90cm, 100cm and 110cm widths. The right one depends on how many people you cook for and how you cook, but it is just as easy to get wrong on the kitchen gap. This tool recommends a width, then sanity-checks it against the space you have measured.
About your cooking
Your kitchen gap (optional, to check it fits)
Measure the narrowest point. Leave blank to skip the fit check.
A range cooker needs a small ventilation gap each side, usually a few millimetres, and the worktop opening is normally cut 2 to 5mm wider than the cooker. Always check the exact clearance in the manufacturer's installation sheet for the model you choose before cutting anything.