Limited to 1MP still captures with OV5642 ArduCam Mini + Arduino Uno
The Arducam Mini Shield is limited to 512kb frame buffer, so photos of larger resolution easily overflow the buffer. You can fit higher resolution photos onto the frame buffer by increasing the compression or adjusting other settings on the camera that reduce the file size of the resulting image (reduce saturation, windowing, etc).

To change the compression, adjust register 0x4407. The higher the number the more compression there is. The camera's default is 0x0C and the Arducam sample code sets it to 0x04.  myCAM.wrSensorReg16_8(0x4407,0x09);

To adjust other settings on the camera (including the ones listed above) check out the detailed list of camera registers at http://www.dragonwake.com/download/camera/ov5642/OV5642-DS.pdf

Also, Arducam is releasing the Arducam Mini Plus which has an 8 MByte frame buffer and is available on advanced order for $35 USD by contacting Arducam directly.