This page explains how to increase the maximum number of tiles rows on the StartScreen of Windows 8.1. This simple procedure can be performed from the Registry Editor.
Open the Run menu and type regedit to open the Registry Editor. Go to:
Click on Edit > New > DWORD (32-bit) Value. Rename the key as Layout_MaximumRowCount. Double-click on the key to edit it. Enter the number of rows you want to have (up to 5-6 rows on larger screens). Close the registry and restart your PC.