Fitting all Tables to the Page Width

VSTO

string mypath = "Document.docx";
Word.Application wordApp = Application;
wordApp.Documents.Open(mypath);
foreach (Table table in this.Application.ActiveDocument.Tables)
{
  table.PreferredWidthType = WdPreferredWidthType.wdPreferredWidthPercent;
  table.PreferredWidth = 100;
}

Aspose.Words

Document doc = new Document( "doc.docx");
foreach (Table table in doc.GetChildNodes(NodeType.Table, true))
{
	table.PreferredWidth = PreferredWidth.FromPercent(100);
}

Download Sample Code