If you record what you're streaming (using the "use stream encoder" encoder setting for recording), you're recording exactly what is sent for streaming. This is usually constant bitrate (CBR), so you can compute exactly how big that file will be:
size = duration * bitrate
For example, if you're streaming 40 hours with bitrate 10000 (which is 10000 kbps):
size = 40 hours * 10000 kbps
which is:
size = 40 * 3600 seconds * 10000 kbit / second
1 byte is 8 bit, so 10000 kbit/second are 10000 / 8 byte / second = 1250 kB/second.
So:
size = 40 * 3600 seconds * 1250 kB/second
size = 40 * 3600 * 1250 kB
size = 180.000.000 kB = 180.000 MB = 180 GB
If you record with a second encoder session (use Nvenc or x264 as encoder for recording with a quality based rate control such as CQ or CRF or simply the simple output mode, instead of (use stream encoder)) that results in a average bitrate of perhaps 50000, multiply the file size accordingly. The actual average bitrate for quality based rate controls depends on the recording settings and on the footage, so you should do test recordings and see what average bitrate actually comes out of the test video.