There are a number of points to consider when installing Linux, especially on Chrome OS Flex.
Chrome OS Flex
Linux needs more than 4GB of RAM to run on Chrome OS Flex. It is a good idea to turn on Intel VT in the BIOS too, though I have not seen that mentioned anywhere. Most Flex users will have at least 256MB of SSD storage, so that will probably be less of an issue.
Disk size
The default disk size is 10GB, which is a good amount to begin with. It is very easy to change it later, so it is should not something you should beat yourself up about. You can share folders with Linux, but that will impact on performance.
What you get
All you end up with is an app called Terminal, which allows you to run Linux commands commands. You do not get a desktop, or any kind of visual interface. This makes it difficult to explore Linux, if you are not familiar with it.