I would like to know how to get the actual size of the picture through the memory method before saving

[cbtan]

cargo.toml
[dependencies]
image = "0.25.2"

main.rs
use image::{imageops::FilterType, GenericImageView};
fn main() {

let img: image::DynamicImage = image::open("C:/Users/t/Pictures/B438097临时/21.jpg").unwrap();
let img = img.resize(500, 500, FilterType::Lanczos3);


//我想知道如何在保存前通过内存的方法获取图片的实际大小，谢谢
//I want to know how to get the actual size of the picture by the method of memory before saving, thanks
 println!("{:?}", &img.as_bytes().len());



img.save("test.jpg").unwrap();

}