As someone who is more than a little Pooh-sized, I have dined at Sci-Fi and LOVED the atmosphere. I felt the food was good in a fun kind of way, but as others have said, you do go for the atmosphere more so than the food (and the atmosphere IS worth the trip).
As for seating, the cars are pretty snug and would be difficult for anyone large or Pooh-sized to sit in. When I went, I had actually lost quite a bit of weight at the time and fit comfortably, but I would NOT try to be seated in the car now...my desire to get back to Sci-Fi is part of the motivation to lose weight again
So I'd say to definitely request the tables. I've seen tables with umbrellas in the rear of the restaurant but I didn't notice the cars that have the regular tables and chairs within them until I saw the picture that Master Yoda posted, so that is nice they have those as well.
The only other suggestion is that you might want to request a table in advance before being taken to a seat...a CM may or may not think about your needs before trying to seat you, and I find it's always better to make such a request in advance before going to sit and then have a problem inside--especially since it's a darker restaurant (and it was completely full when we went).
It is a fun restaurant with an amazing atmosphere that is really unique. The food is decent, as long as you don't have too high of expectations--kind of think 50's road-side diner/greasy spoon food and that is what you'll get (which does fit the theme after all).
Enjoy and have fun!
