The checkbox is an image. The images settings cannot be modified, but you can replace it with your own image by setting the CheckBox item's CheckedImage, UncheckedImage and IndeterminateImage properties - CheckBox item
Our recommendation is to use an image - 96dpi, 16x16, as images are not re-sized when they are used by the CheckBox item.
I hope this helps.
Telerik by Progress
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items