The maximum file size for an Turnitin assignment is 40mb and 400 pages (as at 27/6/17)
A really quick way to reduce a large Word document in size is by saving it as a compressed pdf (see below for a screenshot of Word 2010). Click on FILE > SAVE AS then at the bottom change the SAVE AS TYPE section to PDF, click on the MINIMUM SIZE (PUBLISHING ONLINE) button. Then click on SAVE.
If you have more than one file to submit you will need to merge them into one document (in Word if possible). If you cannot manage this in Word try saving your Word doc as a PDF and scanning the addition items as PDFs and joining them using free PDF joining software such as PDF Mergy which is available as an app from Chrome Web Store (others apps are available).