1. A method for estimating an image size after compression, suitable for an electronic apparatus, the method comprising: receiving a raw image; sampling the raw image for obtaining a sampling image; compressing the sampling image at a plurality of testing parameters of compression respectively for obtaining a plurality of testing sizes after compression; establishing an estimated math model according to the testing parameters of compression and the testing sizes after compression; and calculating a target parameter of compression corresponding to a target size after compression through the estimated math model.