Size box, override width set to width of canvas. If you don’t have one, right click the text in the outliner/hierarchy and choose “wrap with”. Set text to fill. Set justification to “align text center”. Alternatively size to content. Adjusting the size of the text to the size of the canvas isn’t something UE can do dynamically for you I believe, you would need to define an alternate, acceptable option for me to give any help with that. Plus it’s going to look horrible if you stretch out text horizontally or vertically to fit a box so I would steer away from that altogether, even if it is possible.
Auto wrap text. If text is still off the sides you can define the point at which it wraps.