How can i setwidth this table?

PdfPTable mainTable = new PdfPTable(medicineNews.size()+2);

Hello @mygmarjav, welcome to the community.
Please, the next time provide more context in the question otherwise it’s extremely difficult to help you.
With a quick Google search I assume you are using Java with the class com.lowagie.text.pdf.PdfPTable.

Please, if you haven’t already, take a look at the documentation for such a class.
The one I found is here:

Edit: in that documentation there are various methods related to the width

Edit2: also, if it Java and the question is not html-css related, it should be moved to the correct category.