It seems you are asking which video fromat produces the smallest file sizes.
The simple answer is the one using the loweset resolution: 4. WVGA 848X480.
The degree of compression will also affect file size (and image quality). Higher comression gives a smaller file size but lower quality.
Page 5 of the manual tells you how much data is generated per hour. You DID try looking there, didn't you?
See also page 3. You shouldn't have to worry about filling up the card. You can set Loop recording which lets you set the video duration (1, 3, 5 minutes or off) for loop recording. When the microSD card is full, the device will delete the oldest unlocked video file automatically.