It is essentially arithmetic. If you know the project duration and the bit rate you are going to render to, then you can calculate the size. You need to allow a bit more of course for audio and menus.
At 8 Mbps, the max you should probably use, you can fit about 1 hr 15 min. At 6 Mbps, you could fit about 1 hr 40 min.
m i c k,
in future PeterDuke's advice will be more useful to you. The data storage size of a video is determined by the bitrate.
Google 'bitrate calculator'.