X11 forwarding font too small HiDPI HDPI

export GDK_SCALE=2
export QT_SCALE_FACTOR=2
export ELM_SCALE=2

Or
Add, e.g. GDK_SCALE=2 before running an app.
For example GDK_SCALE=2 nautilus

For eclipse, add GDK_DPI_SCALE=2 before running it.
For emacs, add GDK_DPI_SCALE=2 before running it. Also the font size needs to be changed to: height 205

For Filezilla, I have to install Filezilla 3.25 from source code. There are a bunch of dependencies missing because I'm using Ubuntu 16.04.
Important libraries to install:
sudo apt install libgtk-3-dev
Manually installed libfilezilla (version 0.9.1), libgnutls (version 3.4.15), and wdwidgets (version 3.0.2).
Once Filezilla uses GTK 3.0, it can be launched with GDK_SCALE=2 in front of it.
For example:
GDK_SCALE=2 filezilla

For Ubuntu 16.04 Chrome:

4. sudo gedit /usr/share/applications/google-chrome.desktop
5. Find this line:

Exec=/usr/bin/google-chrome-stable %U

6. Change it to this:

Exec=/usr/bin/google-chrome-stable --force-device-scale-factor=2 %U

7. Save/close, relaunch Chrome.