Setting the width and heigth causes it to occupy more space, but the checkbox itself is still small. I've tried both GUISetFont and GUICtrlSetFont with no success, even though the help file says: Thanks for any help.
You can use the font 'Marlett' to have the original characters.
$hGUI = GUICreate("Test", 200, 150)
$hCheck1 = GUICtrlCreateLabel("", 10, 10, 20, 20, 0x1201)
GUICtrlSetFont(-1, 19, 400, 0, "Marlett")
GUICtrlSetBkColor(-1, 0xFFFFFF)
$hLbl1 = GUICtrlCreateLabel("A user-drawn checkbox", 35, 15, 200, 20)
$hCheck2 = GUICtrlCreateLabel("", 10, 50, 25, 25, 0x1201)
GUICtrlSetFont(-1, 24, 400, 0, "Marlett")
GUICtrlSetBkColor(-1, 0xFFFFFF)
$hLbl2 = GUICtrlCreateLabel("Another user-drawn checkbox", 4