Measurements for most items are detailed in the descriptions, in inches. In addition, photographs are taken with examples of things inside.
Project bags are also noted by how many skeins they will hold. The skeins I have used are typically Worsted weight and unballed. This means that if you want to store yarn balls in a bag you’ll likely get more in than I have quoted. However, if you want to assess capacity for your project then a bag that takes two unballed skeins will roughly equate to a two skein finished project, giving you space for yarn and your work in progress.
Please contact me if you are unsure - I knit and crochet myself and can answer most related questions :)