setsize number

This command sets the size of the turtle and requires a number. 

If you have added clipart to your turtle, the clipart will shrink or grow.

The original size is 40. 

The minimum size, for visibility, is 5.

The maximum size is 160. 

Type each of these commands in the Command Centre and press Enter/return:
(You must have a turtle on the page)
setsize 10 

setsize 100
repeat 10 [setsize 10 wait 5 setsize 100 wait 5]

setsize random 160