Excel comes with a standard cell width (64 pixels) but often content is added that exceeds this width. To resize cells to capture the entire content double click between the letters on top of the columns. You will need to click on the line to the right of the column you want to resize.
The exact same method can be applied to resize the height of cells to capture multiple lines of content.
To resize the whole worksheet to capture all content in all cells highlight the entire worksheet (click on the left top corner between the number 1 and the letter A) and then double click between columns A and B.