While you cannot access the check box itself, to increase its size, you can change the font, font size, font color, fill color, as well as add a drop shadow to it.
Right click the check box and select Properties.
To change the Font:
Scroll down to the Font property and click the currently listed font.
An ellipsis will appear. Click it to bring up the Font Window.
Select the Font, Font Size, Bold, etc and click 'OK'.
To change the Font Color:
Scroll down to the ForeColor property and click the drop down menu.
Select the Palette tab and choose the color you wish the text to be.
Click 'OK'.
To fill the Check Box with color:
Scroll down to the BackStyle property and select 'Opaque' in the drop down menu.
Scroll up to the BackColor property and click the drop down menu to the right.
Select the color you wish to use and it will be immediately applied to the Check Box.
To add a drop shadow to the Check Box:
Scroll down to the Shadow property and select 'True' in the drop down menu.
To center the Check Box text:
Scroll down to the TextAlign property and select 'fmTextAlignCenter' from the drop down menu to the right.