I removed something and can't get it back

I’ve recently started using Toon Boom Harmony Essentials and have removed the a box on the right side that can change the thickness/thinness of your brush/pencil tool. I don’t know how to get it back, and haven’t found any answers on the forums after looking for a solid half hour. Please help!

That sounds like the Tool Properties window. It is context sensitive and changes depending on which tool you have selected. It is not exclusively for the brush/pencil tools.

To bring it back:

Option A: Top menu bar: Windows => Tool Properties

Option B: In the top-right corner of an open window (windows are also called “Views” in Harmony) click the plus sign and choose Tool Properties from the list.

See “How to add a view”