I currently dont have Illustrator but I do have Photoshop, is it possible to open a .ai vector file and save as JPG/PNG in a desired size?
Yes, .AI files will be rasterized when you open them into Photoshop. You'll be provided with a dialog asking you for the dimensions you'd like to render the file pixels at:
You could alternatively place an .AI file in an existing Photoshop Document using File ▸ Place... Using the place method, the .ai file is retained as a vector smart object within Photoshop.
You can also see a list of the types of files that Photoshop can open by opening up the "Files of type" dropdown box