Expanding on Paddy's good information, I have placed, in the first column cell of a given row, the formula =CELL("WIDTH",A1), where A refers to the first column whose width is desired. Then, I just copy that formula across the row, as far as I need, and, the column "widths" become visible.
However, be advised that you only get the rounded-up integer value of the width. Thus, if the width is 8.44, you obtain 8, and, if the width is 8.56, you obtain 9.
So, if you are needing to get exact values, the only way I know of is to put the mouse cursor at the meeting together of two consecutive column letters, say B and C, to read the width of column B. Then, write down the value read, and, so on.
Now, if you mean to format your columns so as to get a sheet that will fit on a given size paper sheet, the most practical way is to simply set your print area, then, you can see the dashed line that represents the edge of the paper. Now, you may proceed to reduce your columns as necessary to obtain the page width you need to print out.