Paper comes in a variety of sizes. A5 paper measures 8.3 by 5.8 inches, or 210 by 148mm. A4 paper is almost exactly twice the size of A5 paper, with a measurement of 11.7 by 8.3 inches or 297 by 210mm. In other words, A4 paper is close to the size of letter paper; they are the same width, but letter paper is 279.4mm long rather than 297mm. When you are using Microsoft Word, you can easily resize your A5 paper to A4.
Open the Word document for which you wish to resize the paper from A5 to A4. Click the "Page Layout" tab near the top of the Microsoft Word screen. This brings up a list of options across the top of the screen.
Click the "Size" option in the list that appears. This opens a drop-down list of various paper sizes. Click "A4" to resize your document from A5 to A4.
Click "More Paper Sizes" at the bottom of the list of paper sizes if A4 does not appear as an option. This opens a new window. Select "A4" from the "Paper Size" list or manually enter the size of A4 paper (11.7 by 8.3 inches). When you are done, click the "OK" button to resize the paper.