It depends on the phone you are using though, each phones have different methods on how to do this. Like on my phone, if i want to change the front size i simply need to go to the phone settings then click on display and brightness. There is an option there for me to change the size of the font. Then in terms of the colour of the screen affecting your eyes, its either you sort this out by reducing the brightness of your screen, or you can simply on your eye care option. These days most phones comes with this option, you can easily find it at the push down notification menu. If yours does not come with it, you can also download it on your app store.