On our page we have 2 files to download. One is .ZIP, another .DMG The problem we have is when you download .zip the browser will show you file size, but when you download .DMG it will not.
I know this is not wordpress problem but I hope that someone here has an idea what is wrong.
I hate doing this.
1) It's not WordPress.
2) It's not a problem, it's how browsers and servers work.
See, your browser can calculate file size based on information from your server, which is why they tell you what the size is when you download a zip. I suspect your server doesn't do that for DMGs, which are less common than exe and zip. Talk to your server tech, but there may not be a solution.
Meanwhile, this site is for WordPress support, and alas I have to close your post. Good luck!