There is no specific definition of a "big file". Every service or platform has its limits. With this app you can change the dimensions of a photo with the smallest possible file size! All you need is the dimensions (width & height) of the photo you want to change in pixels.
For your photos to always have the best look, be SEO friendly and small file sized (megabytes), you need to upload your photos to the dimensions suggested by the platform or the developer of the application you use.