Be aware that the optimal insert size depends on a combination of factors, including the sequencing kit (refer to sections on read type and read length) and intended application.
Additional constraints, such as sample type, can prevent selection of an optimal insert size. For example, DNA derived from FFPE or from plasma may already be fragmented below your desired insert size. In these cases, it’s important to choose compatible library preparation methods and to adjust other parameters, such as number of cycles and loading concentration, to achieve a high-quality run.