There are some steps to take:

1. System wide Menu and titlebar scaling

Starting with Ubuntu 14.04 we have an option that helps a bit:

Scaling Support

open the System Settings (here in english:)

LANG=c unity-control-center

Go to "Displays" and set the "Scale for menu and title bars":

Since Ubuntu 17.10 the scaling can be set in

LANG=c gnome-control-center

Go to Settings > Devices > Displays there

see also: How to find and change the screen DPI?

2. Universal Access

Go to "Universal Access" (unity-control-center universal-access) and select "Large Text".

Note: not all applications handle this correctly, some will not reserve the extra space, so some UI elements are not accessible with this option!

3.increase unity dock size

In unity-control-center->Appearance->Look at the botom, you can adjust the size

4. adapt Firefox

see: Adjust Firefox and Thunderbird to a High DPI touchscreen display (retina)

(or use Chrome, which works fine since Version 41.0.2272.76 Ubuntu 14.10, though Chrome will need to be restarted to take effect)

5. increase font in Pidgin

There is a plugin you can install

sudo apt-get install pidgin-extprefs

Then you can increase the font in Plugins->Extended Prefs

6. create starter for applications that still don't scale

Some applications still don't obey the global scaling (mainly java) for those few applications you can create a starter to only Fix scaling of java-based applications for a high DPI screen

in older Ubuntu versions, with unity-tweak-util in the section "Fonts" you can set the "Text Scaling Factor" to 2.0. This will scale the fonts in most applications to double size.