Is it possible to make the TOC expand and collapse icons larger?

You can use graphics that are slightly taller, but not wider.

If you go to this page and take a look at the example named Supply Chain Management, you will see that I have used a different TOC expand image that is slightly taller:

http://www.infosemantics.com.au/portfolio

I had originally tried to use a larger image but it appears that Captivate limits the width.

You can use the On Success action of a button to assign the value of the cpCmndTOCVisible system variable to 1 or 0 to open or close the TOC.  You can create a toggle action using a Standard or Conditional action to make the same button alternately open or close the TOC each time it is clicked.