The contents page of a biography or memoir will give you an idea of the structure the author has used. Chapters in biographies are usually around 6-8,000 words in length but there are no hard and fast rules. The occasional short chapter can bring variety to the pace of the book. Some authors punctuate their chapters with short asides in which they pause from the main narrative to expand a particular theme without interrupting the narrative flow. Kathryn Hughes's The Short Life & Long Times of Mrs Beeton is laced with "interludes" in which she explores topics such as Mrs Beeton's awareness of the link between health and diet, and whether Mrs Beeton ruined British cooking. Each adds to the reader's knowledge of the period without causing the narrative drive to stall. Alternatively, you might prefer to weave themes into the central story.
Cradle to Grave
A biography typically starts with the subject's birth (it's surprising how many begin with a description of the weather) and continues in a roughly chronological order until their death. The advantage of this approach, which could equally apply to the history of a family, is that it is easy to follow. The downside is that it can appear plodding - especially if you're writing about someone who had an action-packed early life but whose later days were tame. One solution is to condense your treatment of the less exciting years but this can be difficult to achieve without making the book seem unbalanced. Alexander Masters turned the traditional form on its head in Stuart, A Life Backwards by telling the story in reverse chronological order.