Is there a way to control the app and notification volumes at once on a phone like there is on a tablet? Whenever I want to change the volume for apps on my phone, I have to open up an app that makes sound. If I try to do it at other times, it only changes the notification volume with no option for others.

Like this page says, this is an option for tablets, where you can just expand the volume controls when they pop up and then adjust either one. Is there a way to get this expanding functionality on a phone?

I guess that they made it like that to save screen space, but neither a button nor a slider bar comes anywhere close to filling up my screen, and I would appreciate being able to do this.

I have a rooted Nexus 4 with Android 4.4.2 and have the Xposed Framework installed.
That depends on your Android version and device, but based on what you said it seems like you should be able to. You can try Volume in Settings, or you can download Volume Control and control various different volume levels and settings.

Several attempts to get this are posted here, but it doesn't seem like some devices have it.