To change the row header width it is necessary to modify style settings for all four parts of the grid. Here is the example how to set a row header width to 100px:
<style>
.active-scroll-left, .active-scroll-corner {width: 100px}
.active-scroll-top, .active-scroll-data {padding-left: 100px}
</style>
Changing the column header height is very similar:
<style>
.active-scroll-top, .active-scroll-corner {height: 25px}
.active-scroll-left, .active-scroll-data {padding-top: 25px}
</style>
You can do the same thing in javascript using the methods of the grid object:
obj.setColumnHeaderHeight("25px");
obj.setRowHeaderWidth("100px");
Example: