Micro Tutorial about manipulating text.
This is the tip of the iceberg of text manipulation.
I'll just show the bare basic manipulation, adding text strings together.

What I do is add This Cube is (size of cube) Cubic Cm.

Follow up:

Well it is extremely simple. Just use a Math Add node set to String mode. You can make as many inputs as you wish.

This method can be used to make the formula spline do what you want too.

And Add User Data fields in the equation.

That is it really.