Greetings Shirley:
Thank you for showing a keen interest in the Illinois Big Tree Register. As you probably already know, only the largest individual tree of a native Illinois species is listed on the state Register. "Largest" is a function of three precise tree measurements (1) circumference, (2) vertical height, and (3) average crown spread.
In order to best answer your question and to clearly articulate, "How to measure big trees", I strongly suggest visiting the Illinois Big Tree Register website http://web.extension.uiuc.edu/forestry/il_big_tree.html
Please scroll down the web page to the heading "Illinois Big Tree Measurements", where you will find detailed descriptions and images on how to measure circumference, height, and average crown width.
Again, than you for your question and your interest in the Illinois Big Tree Register.