Access doesn't provide you a way to enlarge the check boxes. Sometimes this just isn't good enough. Sometimes you need or want check boxes that are larger than the default. Here's a simply way to do it.
Create a text box. Name it chkUnchecked. Set the Locked property to Yes. Set the font size to 18 (or whatever works for you).
Copy this text box. Name the copy chkChecked. Put a capital X in this new box. Set the Visible property to No. Place this text box directly over the first one, so the two are on top of each other.
Now, the code to make it all work. In the AfterUpdate event of txtUnchecked, put:
That's it. You now have a check box that is bigger than standard. You have to remember to evaluate this later as needed. You will have to check to see which of the two text boxes are visible and set the checkbox field in your table or whatever as appropriate.