How to reduce Image Size and Dimension - go

Question

I want to process local images to make the dimension as Height = 380 PX, Width = 380 px and size is less than 20kb.
I have the following code which reads the file and then change dimension to 380 by 380 and then try to reduce the size.
But size is sometime more than 20 KB if original file is large.How to make sure that file is always less than 20 KB?
how to check size of image after dimension change and size after quality reduced image ? so that I can run a loop.
package main
import (
"bufio"
"bytes"
"fmt"
"image"
"image/jpeg"
"image/png"
"os"
"github.com/davecgh/go-spew/spew"
"github.com/nfnt/resize"
)
func main() {
// Read Local file
mainFile, err := os.Open("C://c.jpg")
if err != nil {
fmt.Println("Erro Occured")
spew.Dump(err)
return
}
defer mainFile.Close()
originalimage, _, err := image.Decode(mainFile)
if err != nil {
fmt.Println("Error ", err)
originalimage, err2 := png.Decode(mainFile)
print(err2)
print(originalimage)
}
// Create new file with reduced dimentions - height width as 380
newImage := resize.Resize(380, 380, originalimage, resize.Lanczos3)
// Create new file with reduced size - Quality down by 50%
var b bytes.Buffer
w1 := bufio.NewWriter(&b)
jpeg.Encode(w1, newImage, &jpeg.Options{50})
f2, err := os.OpenFile("C://temp5.png", os.O_WRONLY|os.O_CREATE, 0666)
if err != nil {
panic(err)
}
defer f2.Close()
b.WriteTo(f2)
}