You can set column header height with CSS:
<style>
.active-scroll-top, .active-scroll-corner {height: 50px}
.active-scroll-left, .active-scroll-data {padding-top: 50px}
</style>
or with js method call (which sets the same CSS rules):
obj.setColumnHeaderHeight(50);
If you are using xp skin you may need the header border height bug fix.