We moved up from our 26' with 1 slide to a 30' with two slides. Having 3 dogs was a huge factor in our decision. We had 1 when we bought the 26' trailer, but after a ton of use and a couple more additions to the family, it was time to upgrade. We needed lots of floor space! We love the opposing slides and my favorite part of the set up is that I can get to the bathroom, bedroom, refrigerator and pantry without any of the slides being out. That makes things a lot easier if you need to pop in the trailer during a long trip or need access to things with no slides out.
Personally, I think if you have any dogs and no slide you'll be unhappy in the long run. Based on your situation, I'd go for about a 24ft with 1 slide. I think layout trumps length though for appealing to your personal needs.