The height a particular row is controlled by the method tableView:heightForRowAtIndexPath:
of you UITableViewDelegate. You cannot manually change the size of a cell, it is resized by the TableView to the cell size (tableView width and row height) when it layouts its subviews.
On another point, what it the point of this line:
newSize.size.height = UI_USER_INTERFACE_IDIOM() ? 48 : 48;
It sets the same value and compares the idiom to 0 instead of iPhone or iPad (one of them might have the value 0, so the check might be working for now, but using the constants is better).