I tend to agree w/ Sealcove - I think that an 18-20" bar is a good size for most of us, most of the time, in the wood we get most often... I bought my 7900 Dolmar because I wanted a saw that would be big enough to handle ANY log that I got, including the occasional big monsters - but it's way overkill for most of my cutting, a Dolmar 5100 would have been a better match for 90% of what I do. (and if my Pull-on dies, the 5100 will be on the short list to replace it)
I have a 28" bar to use on my Dolmar, but I only put it on if I have a log to cut that's over about 30-35" around, which has happenned about twice in the last year that I've owned the saw - I have done about ONE tank of gas cutting with the 28" (and part of that was just running out the tank before I switched back to the 20") and about 10 tanks with the 20" bar... With the 28" bar, it's a great performing saw, that is heavy, tip heavy, clumsy and awkward to use. I would not want to cut much with a 28" bar. With the 20" bar, it's wonderfully balanced, with reasonable weight, and performs even better...
Gooserider