When you bump into an unowned force, you will be given its descriptive force size. The size represents non-leader units only. Items and leaders are not counted. You could have a group with 20 Wizards, 10 Magic Items and 1 Human Militia and it will be seen as a Small force. When you receive a size description of a force, they range from:

Insignificant	0
Small	1 to 2
Sizable	3 to 5
Large	6 to 10
Great	11 to 20
Vast	21 and up