"Translate" mobile app mockup dimensions from desktop to smartphone size?

I'm pretty sure this is a common issue, but I'm not certain if I'll be able to explain myself correctly. So please bear with me.

I'm including a screenshot of a mobile app I'm working on in my Windows 10 PC. For the sake of argument, let's say I created it in MS Paint.

As you can see, in Paint the font is 48pt and the textbox is 580px wide. On the other hand, if I email this "drawing" to my iphone and look at the drawing from the phone, the image looks perfect. I know the picture is resized in the phone.

On the other hand, I sent this same picture to the developer, and the font in the app that he deployed (in my ihpone 7) is too big, but I don't know what size I should tell him. The same thing goes with the other controls. I assume that he uses an iphone plus, while I have a regular iphone. I assume Android will have the same issues.

My question, How do I tell the developer when he asks me about font size and control size? I can't tell him that the font is 48pt because I assume it'll be too big for the iphone.

In other words, how can I "translate" the sizes in my Windows 10 PC to sizes that would fit a smartphone?

The reason I ask is because I sent a developer a mockup of a screen that I created in FireWorks, but the font sizes are way off. So I need to tell him the specific font sizes, but I don't what those sizes are.