I don't know of a way to generate random text in Publisher, or why you want to, but thought I would offer a couple thoughts.
Publisher text boxes are independent of the text or content within them. If you go to View > check Boundaries, the boundaries of text boxes are visible even when there are no contents/no text. If you don't check Boundaries then the text box is invisible
on the page unless you mouseover it. I am not sure why the default is set this way, but it is. My point is that if you mark boundaries and draw a text box on the page, it will serve as a placeholder just like an empty picture box, without any text in it.
Further, the size of text boxes are independent of the quantity of text within them, by default. If you key, paste or import too much text into a text box you will see an overflow box that allows you to link/flow the excess contents to another text box,
perhaps on another page in the publication. Alternatively, right click the text box > format text box > text box tab > you will see that you have the option of changing the way text autofits: do not autofit (the default), shrink text on overflow, best fit
and grow textbox to fit. (these commands are also on the ribbon if you prefer)
You did not specify why you wanted to generate random text. Perhaps this extra information will help.
DavidF