We haven't tried an 8-hour recording in My Screen Recorder Pro but a 2-hour recording takes close to 3GB with default settings. However, if you want to record for 8 hours, here are some ideas to reduce the total recording size.

1. Reduce the recording time to around 1 frame per minute (If your captured screen is going to have low levels of activity).

To reduce the frame rate:
a) On the Tools menu, click Settings.
b) Select the AVI Video tab.
c) Move the slider under Frame rate to your new frame rate.

2. Turn off audio recording (record only video) if you do not need it.

To turn off the audio:
a) On the Tools menu, click Settings.
b) Select the General tab if not already selected.
c) Clear the option "Record audio".

3. If Audio is turned off then you can also choose a different video compressor that offers higher compression ratios.

These three steps can reduce the recording file size by more than half.