Change the size of a check box inside a tree control

Hello everybody, I have a cvi tree containing check boxes as cell types. The problem is that at small display resolutions (e.g. 1024x786) its hardly possible to recognize if the box is checked or not, because the check mark only consitst of a few pixel. I tried to increase the default cells font size, but it seems to have no effect on the size of the check boxes. Does anybody know how to increase the size of a check box inside a tree control? Thanks, Marlon