You might take a look at some of the examples found in "Adventures in FileMaking #2 - Enhanced Value Selection". There are "check box" controls there where the check box is not really a check box but either conditionally formatted text or a graphic image--either option might work as a way to make the check box part of the control larger.
And I know of now way to make the text inside a pop up menu larger except by specifying a larger font size for the field you have formatted with the value list. I don't know if there is an issue here with Web Direct or not when it comes to the font size, but if all else fails, you might take a look at some of the alternatives in this same file as ways to present the user with a list of values where you have more options for how the text listed as a value to select is formatted.
I am in the same situation here now. Is there any better way to handle the checkbox sizing? If checkbox can size proper in Webdirect would be a plus.