I found the solution here: https://askubuntu.com/questions/1244711/thinkorswim-installer-too-small

Changed the display scaling to 100% then ran the installer.

Instructions for Lubuntu / Ubuntu 19.04

Thanks to Michael in tech support at Thinkorswim.

The following instructions should help you to install thinkorswim with the proper version of Java:

Installing Zulu OpenJDK:

1. Log in as root or use sudo

2. Import Azul's public key

   $ sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 0xB1998361219BD9C9
   

3. Add the Azul package to the APT repository

   For Ubuntu

   $ sudo apt-add-repository 'deb http://repos.azulsystems.com/ubuntu stable main'
   

   For Debian

   $ sudo apt-add-repository 'deb http://repos.azulsystems.com/debian stable main'
   

4. Update the information about available packages.

   $ sudo apt-get update
   

5. Install Zulu by using the following command:

   $ sudo apt-get install zulu-8
   

Once Zulu is installed move on to installing ThinkorSwim:

1. Navigate to downloads and right click "thinkorswim_installer.sh", select "Properties"
2. Select the "Permissions" tab, and make sure "Allow executing file as program" is selected
3. Close the window
4. Double click "thinkorswim_installer.sh"

-IF ONLY A TEXT EDITOR OPENS, CLOSE IT AND CONTINUE BELOW-

5. Click back onto the file explorer
6. Open from the top menu bar: Edit > Preferences OR Files > Preferences
7. Select the "Behavior" tab
8. Select "Ask each time" OR "Ask what to do" under "Executable Text Files"
9. Close the window and launch "thinkorswim.sh" and select "Run" from the prompt