M+S stands for Mud+Snow, indicating that the tires are suitable for use in the winter (studded or non-studded winter tires). The official winter tire marking is the snowflake symbol, also known as the 3PMSF marking.(the techinical name is 'Servere Service Emblem".
By law, a winter tire must contain at minimum the M+S marking, but in practice, for harsh winter conditions the tire should also have the "three peak mountain symbol" or "Severe Service Emblem" on it.
The size markings of tires that are compatible with your car are shown on the manual of your car. The size is always marked on the side of the tire. Several size options are available. Size markings may be in the form 205-55 R 16 94 V XL.
Your vehicle registration will note what speed class is suitable for you. Car manufacturers determine tire size and speed class to make sure the tires can accommodate the top reachable speed. The speed class is indicated by the second-to-last letter in the tire marking: In a tire size 205-55 R 16 V …
Always follow your vehicle manufacturer’s instructions when choosing a rim size.