A play date, as in someone else's dog might be good for your dog as far as getting some energy burned off but I personally do not think you should get another dog right now. If you add another dog to your hosuehold right now I am afraid you have a better chance of having 2 uncontrolable dogs instead of one. Bad habits are easier to pick up than good habits, and your dog may very well teach the new dog bad habits.

but in general, just like any other dog, as long as they are socialized properly they will get along with other dogs. Size doesn't really matter to them, just depends on how they were socialized.
Since you do not know any background on your dog I'd find someone with a laid back dog yet one who is of decent size so that (s)he won't get too hurt just in case your dog acts agressive. Make sure you tell the other dogs owners that this is the first time you have introduced a dog to your dog so you are not exactly sure how he will react.