It is definitely a Sony sensor, thought to be the previous-gen IMX230 ( same as in the Experia Z4 and iPhone 6). So that's 1/2.4 inches, or 5.92x4.57mm. Pretty much standard fare for Chinese mid-tier Mediatek phones.

For comparison the famous Nokia 808 had a 1/1.2 inch sensor, 10.67x8.00mm.