I´d say that it mainly depends on how tall you are and how far can you reach (without stretching yourself). Cause to do a smooth slide you can´t move your legs or change position in any way.
I went for the 80cm Konova slider for this particular reason and I really don´t regret it. 80 cm is enough, if you need more then 80cm in one shot it is likely you probably need a lot more, like 2 meters and above and that´s not a job for a slider.
I'd have to say that Trixter pretty much nailed it here. It's all about preference & the needs of the shot, but I think 80cm is a great settle. I run a 75cm slider, and I like it a lot for local shoots, but wouldn't go any bigger. Like you mentioned, portability is a big deal, and you may be surprised how little of the actual slider length you really use. Not to mention trying to get through all your setups with a two tripod setup... Man, that just kills your set up time.
In fact, I've actually been very interested the Edelkrone slider that Photoshopper mentioned, just because it's small and portable and I'd love a slider I can fit in my carry-on. (Which means it couldn't be any bigger than 21"-22").
In the end, just play with whatever models you can get access to and pick the one that has the options, length, stability, and price that fits you and your technique best.
Be interested to hear what you decide on.