First step is to measure the “eye to eye” length in mm (e.g. 190mm) typically taken from the centre of each fitting location, then the “stroke” which is how far the shock can travel/compress (e.g. 60mm). This will help you find the appropriate size you need. Getting the right size is important as shock will be szed according to the dynamics of the frame, and it could put you on a completely weird angle and cause parts of the bike to hit each other.