I have been out to eat with many large service dog teams. The largest dog I have been out to eat with is a 250 lb English Mastiff. We had zero issues. I will post pictures below of meetups with Ollie (28 inch Standard Poodle), Iris (similarly sized GSD), and Zeus (250 lb Mastiff) or Gracie (Great Dane).
Scroll to the end for pictures at Wendy’s. Note how Zeus the Mastiff is blocking a not very used aisle. Totally fine. Notice how Iris is not under the table and not blocking any aisle. https://goo.gl/photos/uBWTVLxSwSEBsMUx9
JellyBelly factory meetup, scroll to the middle for restaurant pictures. Notice how Ollie, Iris, and Zeus all are not under the table (except Iris in the beginning). They are all next to our chairs and still remain either not blocking an aisle at all, or only partially blocking an aisle. https://goo.gl/photos/b3J2zRMpTpJBeHAQ6
At a small cafe (scroll to the end) after a meetup. Notice how there is no under table space. Most dogs are either in front of us or to the sides of us. Again, not really blocking any aisles: https://photos.app.goo.gl/1CznFubLGrb67yML8
An Amtrak trip, pictures of restaurants are in the middle. Note how in here, Zeus is positioned against a wall so he can take up a lot of space and no aisle is impinged upon: https://goo.gl/photos/LHJmMZwpcxqbD4ju6
A meetup at Mr. Taco (maybe our first time there?!). You can see all the dogs are either next to or next to and slightly behind our chairs. No aisles are being blocked: https://photos.app.goo.gl/uSFyEaA8XcxXyeDb6
Mr. Taco with Zeus against a wall, and Iris next to a chair. Ollie is under the table this time. Still no aisles being blocked. https://photos.app.goo.gl/J8HhLXo6GiWFdci6A
Ikea meetup, at the end there are pics in the restaurant. Notice how Ollie (poodle) and the Dobie are both next to chairs. Not blocking the aisles. https://photos.app.goo.gl/Q9pzFk7JZaUS6fvr6
I hope this has helped show how large breed service dogs can fit at restaurants, even when they can’t be underneath a table!