Are you seeing that only "Medium" is available for the image size; "Large" is not available?
There are two common scenarios in which you won't have Large available:
1) You're in Sports Finder Mode. This is found in the SHOOTING SETTING menu. Large is not available when Sports Finder Mode is enabled.
2) You're using an electronic shutter (ES) burst mode that crops the image so the buffer doesn't fill up as fast during a burst. You'll press the DRIVE button on cameras that have a DRIVE button. In other cameras, go to the SHOOTING SETTING menu > DRIVE SETTING > CH HIGH SPEED BURST and select an option that does not show "CROP".
If neither of these fix the problem, please send us a photo of your LCD (using a smartphone is fine) so that we can further troubleshoot.