How to get directory total size?

Using a global like that at best is bad practice. It's also a race if DirSizeMB is called concurrently.

The simple solution is to use a closure, e.g.:

func DirSize(path string) (int64, error) {
    var size int64
    err := filepath.Walk(path, func(_ string, info os.FileInfo, err error) error {
        if err != nil {
            return err
        }
        if !info.IsDir() {
            size += info.Size()
        }
        return err
    })
    return size, err
}

Playground

You could assign the closure to a variable if you think that looks better.