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But for everyone that doesn't have Android N (which is basically 98% of us or so), there are other ways to change your DPI (to make all things - not just text - bigger or smaller according to your preference).
So first, you have to make sure that you have Android Debug Bridge drivers for your phone installed on your computer. (It may be a bit of a hassle to find those. If I'm not mistaken, the OnePlus One uses the Samsung drivers. Could never get those to work on my old laptop but got the drivers for my LG Optimus G to work just fine. You may have to switch to PTP mode in your USB settings on your phone to get things to work properly.)
If you don't want to install the Android SDK (You can use ADB to test Android apps that you build. It's a great developer tool.) you can install this: [TOOL]Minimal ADB and Fastboot [4-27-16] | Android Development and Hacking
Then go to developer settings on your phone and Enable USB Debugging.
Connect your phone to your computer. You may get some type of permissions dialogue box on your phone (I don't remember getting one on my LG but that was also a couple years ago).
In command prompt, type
adb shell wm density 440 && adb reboot where you can insert any DPI number in place of 440 that I wrote in the example.
DPI is a measure of your display scale and stands for Dots Per Inch. The things is, while your screen might have a certain Pixels Per Inch (PPI), your DPI and PPI don't have to be the same. Basically, if you lower the DPI, things on screen will get smaller. If you increase it, things on screen will get bigger. My OnePlus One was shipped with a default DPI of 480 even though the screen PPI is actually only 401. I didn't realize how unnecessarily large (to me anyway) things were till I changed the DPI to 400. I essentially get more real estate because everything drawn on the screen is smaller.
HiDPI screens like the Retina MacBooks and Surface devices or my Dell XPS 12 with 4k Display basically pack the pixel density really high compared to regular screens (the PPI is very high) and then scale the DPI. In the Retina MacBooks, the default setting is 200% scaling (this percentage is a linear percentage, not an area), so in essence compared to the standard way macOS would render everything, it makes everything twice the number of pixels wide and tall (thus something that would have taken only one pixel before now takes up 4). This results in a crisper rendering of everything (as long as the images are vector graphics or high enough resolution), especially text. On my Dell XPS 12, I have set my scaling to 250% so that things are readably sized.
If you want to reset your DPI to the default that your phone came with, enter
adb shell wm density reset && adb reboot.
This may break some apps like Android Pay (which I'd like to use but not as much as I'd like to not stare at oversized Icons and text all day every day) and cause some glitches in others (for example, in the HTC Sense launcher that I use, the folders are smaller than they should be and cutoff half of the second row of icons in the folder preview. Everything works fine if you tap on the folder and open it.) so be forewarned.
side note: another great tool that you can use using ADB is the app Helium Backup which will allow you to backup all your apps including app data and settings via ADB even if you don't have root on your phone. I highly recommend it especially for those who are planning to switch phones (to the Axon 7 )