CHANGE THE INTERFACE SCALE THROUGH THE INTERFACE OPTIONS
You can easily change the size of the user interface in Planet Coaster by going to 'Gameplay settings' and un-checking "Interface Auto Scale" under the Interface tab.
You can then move the slider to change the scale of the interface.
CHANGING THE INTERFACE SCALE BY EDITING THE CONFIG FILE
Alternatively, you can change the scale to whatever you like by editing the config file found here:
C:\Users\%userprofile%\AppData\Local\Frontier Developments\Planet Coaster\Config_Local\Game_local.config.xml
We would only recommend changing the value from 0.7 and 2.0. Setting the value higher or lower than this range may cause the interface to become unusable.
If you do run into any interface issues as a result of this change, you can safely delete this config file and a new one will be created with the default settings next time you start Planet Coaster.