I think I understood the question very well:
2. Can i draw something in 100x100 pixels and keep the ratio of it when viewed in the iphone5, WITHOUT it changing?The answer to this question is, as I said, yes, it keeps the ratio, it doesn't change at all.
But that's not what you wanted to know. The question was wrong. So if you make the right question you'll get the right answers.
Now to your new question:
If i draw something in 100x100 pixels. Let's say it is 3 CM. Once important under the retina display it will still be 100x100 pixels but WILL it become smaller, to the eyes compared to when i draw it on photoshop on PC?How many cm a digital image will cover depends on its ppi or dpi value. For digital media ppi is the right measurement, for printing it is dpi.
ppi means pixel per inch and desrcibes the pixel density in relation to its size of a digital medium, like a display. Under Windows you have the option to use several pixel densities, but the most common is 96 ppi. With such a density, a 100x100 pixel image will cover 1.04167 real world inches (~2.64583cm)
The iPhone 5 display is said to have a density of 326 ppi, so a 100x100 pixel image will cover 0.3067 real world inches (~0.7791 cm)
IMPORTANT: The image doesn't shrink! It just looks smaller to the eye. It keeps the same amount of pixels.
I'm no expert in Photoshop, but in GIMP there's a viewing option called "point to point", which makes sure, every pixel of the image fits to one pixel of the screen, when in 100% view mode. If you turn that optin of it will be displayed according to its density. So, if I create a 100x100 image with 326 ppi and deactivate "point to point", I see the image in exactly the real world size, the eye would sense it on the iPhone 5. Since Photoshop is the professional, costly image editing software, I'm sure it has such an option also. I just can't tell you the name, or in which menu it hides. Maybe some Photoshop expert can help here.
So, basically, there is no other way than making them in a higher pixel size, or, as I said in the first post, use a smaller 16:9 resolution an scale it to fullscreen.