Eruvin Chapter 1, Mishnah 6. Our Mishnah deals with the lechayayim - the upright posts discussed in Mishnah 2, in the dispute between Rabbi Eliezer, who holds that two posts are required, and Beis Hillel, who hold that a single post or a crossbeam at the entrance of the alleyway is sufficient. Here the Mishnah comes to establish the proper measurements for those posts.
The measurements of the post in the Mishnah:
"Govhan asarah tefachim" - this is the height required in a post.
"Verochban ve'ovyan kol shehu" - as for width and thickness there is no fixed measurement, and any amount at all suffices, whatever it may be.
What exactly is the measurement of "kol shehu"?
The Gemara investigates how much "any amount at all" is, and clarifies that even a certain type of thick garment that they used to wear in those days constitutes "any amount at all." From here the Rishonim disagreed in understanding these words:
Some understand that the reference is to the thickness of a finger or of a thumb.
And some understand that even less than that suffices, that this is "any amount at all" in its most literal sense - as long as something is there.
The Mishnah continues and establishes that the width of the space in which the post enters the entrance of the courtyard must be three tefachim, and anything less than that is not sufficient.
In summary: This Mishnah establishes the measurements of the posts discussed in Mishnah 2 - a height of ten tefachim, a width and thickness of "any amount at all" (and the Rishonim disagreed about that measure: the thickness of a finger, or even less than that), and a width of three tefachim at the point where the post enters the entrance of the courtyard.