Hi,
If you want to define custom page sizes then you should use igGridPaging.pageSizeList property like this:
//Initialize
$(".selector").igGrid({
features : [
{
name : "Paging",
pageSizeList : [10, 20, 30]
}
]
});

Note: I'm not sure I understood your question correctly, so my answer may be wrong. Please clarify if this is not what you're looking for.

Hope this helps,
Martin Pavlov
Infragistics, Inc