I've done up to 3.5 hour scenario frames, that came out to about between 12 and 18 MB total. There is a slight performance hit but you should only notice it if you're at the very bottom of the scale for hardware.
The problem is going back and reviewing it all. It's really saving positions, instructions, damage, and times, etc. So the film viewer plays back reading the instructions and re-creates events. You're not seeing them as they were in real-time, but it's pretty darned close. That's why the size is so small overall.