The files recorded through OBS are generally 10-20 MBs per 1 minute of recorded video at the default camera resolution and frame rate which is 1920×1080 at 30fps. Recording length is determined by the clinician and resources available on the connected computer.
You can increase/decrease the recording size to match your requirements this by changing the options under the Output tab in the Settings Menu.
Note: Changes to frames per second to be able to play back eye videos in slow motion require a more powerful computer and a dedicated graphics card designed for gaming (eg. NVIDIA/AMD).