Hello @nestor221, I'm afraid your question is not clear. Do you want to divide 51 by 2?
Or are you referring to paper sizes 5.5 and 8.5 inch? In the Margins dialog, you can set the left and the right margins. If you need the margins to be the same size, just use the same number for left and right.
Although your question is unclear, most settings for paper Size or Margin Size are available in the Page setup Dialog Box. You can either access it from the Layout Tab and click on the small square in the lower right corner of the Page Setup group (or by using the shortcut ALT, P, SP