No of Cameras - The number of cameras you have obviously makes a big difference to your storage needs. Just select how many you have.
Resolution - This is the video resolution of your cameras in megapixels. If you have a mixture of say 2MP and 5MP cameras you can opt for an "in between" resolution to give a rough indication of storage requirements.
Frames Per Second - At the expense of a movie quality smooth video, you can pull back on the FPS to say 15 or 12 and save a lot of space. Each second you will be storing the full cameras resolution multiplied by the number of frames - this quickly translates into a huge amount of storage space. Be frugal with your FPS and you can greatly increase your system's capacity.
Compression - This is the video compression scheme where the latest ones tend to be better. If your NVR or DVR has H.265 or H.265+ chose this to get the most out of your storage space.
Quality - This is a measure of the aggressivness of the compression routine. It can make your recordings much smaller at the expense of playback quality or you can opt to have the best quality recordings but use up more space. The middle option normally works for most people.
Scene Activity - On the more advanced compression algorithms if the area being recorded is busy it will take up more space. This is because the compression routine tries to store only the parts of the display that have changed since the last frame. If its constantly moving the compression cannot rely on parts of the picture remaining the same.
Hours Per Day - Most people want 24/7 recording but if you are opting for event triggered recording, set the number of hours you estimate you are likely to record each day. This gives a more accurate indication of likely space needed.