Your monitor is so small?
Any monitor can be used to test, does not need to be a big one.
Under Ubuntu and Windows you can change the DPI settings. On Ubuntu 17 there is an accessibility option. Under Windows 7 or newer you can change the DPI options.
I've tested High DPI apps on a notebook in a small 15' screen.
Lazarus 1.8 does everything automatically, except the values you use by code, in these cases you must scale them by hand using
ScaleX(), ScaleY()
For example if you set the width of a component by hand, you can do
Width := 10
Width := ScaleX(10, 96)
Being 96 the DPI you designed these fixed values.
I added a lot of screenshots on GitHub.