did some recording with fraps something like 2 years ago.
looked up a vid, length is 3:47 & it takes up 23 MB (no sound recordings)
i recall it's very important to set the "fps" rate to the same one NF uses,
otherwise the file will grow very large very fast.. however i don't know what rate it was was again (havent used it on this pc, so the settings were lost)
the old forum holds this data somewhere, pretty sure i looked it up over there back in the day, searching for "recording" or "fraps" should get you going on the right settings i think.