Optimal image resolution is dependent on the size of print in the image. Generally, 1000px on the smaller dimension is recommended for images to avoid blur in images.
Please note, that accuracy is largely dependent on the quality and clarity of the image and file size. The file size limit is 20Mb for direct API calls and 10Mb for Web uploader.
For Images, the image size should be equal to or bigger than 100 pixels (100x100).
Please note, that file compression tools reduce the image size and thus can affect the data extraction accuracy.