A large size photo print will be expensive. Better to use a quick copy centre like Kinkos. Call digital print shops in your area and find out how large then can print. For not much more than a couple of bucks you should easily find 11×17 or 12×18. Even larger shouldn't set you back as much as ten bucks.
Whatever size you decide on, set up your document to that size and scale all artwork. The finished, scaled resolution of any imported images should be 200 pixels per inch or better for best results. Use File > Save a Copy to save a PDF as Press Quality. Turn off Include Illustrator Editing Capability.
Thank you for the sound advice! I haven't had any luck yet finding a reputable online print service in Germany which will print a single file from PDF (their standard offerings are catalogues, brochures, bound photobooks and business cards for businesses). I'm continuing to look and will see if I can find some local print shops which won't charge too much. Thank you for the additional details on how to save the file correctly!
Look for copy centres or business centres, not just print shops. Anyone that has a colour copier probably has it connecter to a RIP and a network, making it a printer, too. All commercial colour copiers can print to 11×17, and most go to 12×18 or 13×19.