If I have a topic title, let's say "My topic is not going to get all displayed in here", the title in the main forum view will get truncated at about 45 characters like so
My topic is not going to get all displayed in...
See also attached pic.
There's a setting called "Extended Layout - Recent post length" but that's once you go inside a Forum. There is no setting to change the Title length.
How do I change it?
You should use the "Extended Layout - Recent topic length" option as well, it's located in the Forums Tab of Forums > Settings admin page: