The question is how to actually include a single quote in the text string without confusing the syntax of the code?
Use a different kind of quotation mark; the single (apostrophe) and the little retrograde accent mark (`) will both work.
VALUES 'MainText' '2"x6"', '2"x8"', ...
It would be preferable for the output to be 2"x6" rather than 2x6.
A bit OT, I would find 2"x8" to be potentially confusing since it should be a nominal dimension. We wouldn't use the inch sign unless it was a true dim. But you should stick with what you and your contractors are accustomed to.