Generally, the page index should be reset when you change the page size of the grid. This can be verified on the following online demo of the grid:
(change the pager mode to 'Advanced' or 'NextPrevNumericAndAdvanced' havigatio to a different page and set page size greater than 100 for example)
Alternatively, if you change the page size manually, simply set the CurrentPageIndex of the grid to 0 and rebind the control invoking its Rebind() method to attain the same effect.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.