I want to specify my own text size in my application, but I am having a problem doing this.
When I change the font size in the device settings, the font size of my application TextView also changes.
Actually, Settings font size affects only sizes in sp. So all You need to do - define textSize in dp instead of sp, then settings won't change text size in Your app.
Here's a link to the documentation: Dimensions
However please note that the expected behavior is that the fonts in all apps respect the user's preferences. There are many reasons a user might want to adjust the font sizes and some of them might even be medical - visually impaired users. Using dp instead of sp for text might lead to unwillingly discriminating against some of your app's users.
i.e:
android:textSize="32dp"