When you start RTW up go to 'options' in the main menu, then 'video settings'. Check the box labelled 'advanced options' and a lot of extra options will appear. There is a drop down menu labelled something like 'unit size', click on the arrow and select a larger unit size from that menu. On the largest setting some units have as many as 240 men in them.

Note: The new unit size will take immediate effect in custom battles and new campaigns, but any existing campaigns you have going will keep the smaller unit size. Having more men in each unit also demands a lot more from your PC, so you may find it gets very jerky.