Choosing a good idea space for a startup involves aiming for a reasonable hit rate for new ideas and ensuring founder market fit. Even if the initial idea is not perfect, being in a conducive idea space allows for exploration of adjacent ideas. For example, the success of 5Tran from YC Winter 13 illustrates the importance of selecting a fertile idea space. Despite initial rejections of their data analysis tool, continuous iterations and interactions with customers allowed them to eventually develop a product that addressed a real need. The story highlights how being in a fertile idea space increases the chances of stumbling upon a successful startup idea, while a bad idea space could hinder progress and innovation.