You can read
part 3 here.
Again, I'm not a huge chromatography expert, I remember some things from college about theoretical plates and column loading, but not well enough to embarrass myself describing them here. What I do know is that the image above, taken from this Dionex app note, very accurately reflects what I've seen when experimenting with column lengths. In this experiment, they used three column lengths, the same flow rates, and the same injection volume. The important part to me from the MS side of thing is the peak height. When the column length increases, the peak height does as well. You can't see it in the above screenshot, but we can assume this -- if we loaded the same amount onto the column and the height increased, that means that the peak width had to decrease. Decreased peak width means increased chromatographic resolution, less coelution effect, and more results for your MS/MS, and this is just with more material. The real way to jack up your results is to increase the chromatographic resolution AND your peak intensity AND your gradient length. You can read about an old comparison of a 10cm 140 min gradient and a 30cm 240 min gradient here. In these limited experiments, I found that I could save time AND increase my coverage by using a longer column/gradient combination. Again, you have to take a good hard look at the requirements of your lab and the study in question, but I hope this helps get you started. As always, if I can answer further questions or if you have suggestions, don't hesitate to contact me or post questions here and I'll do what I can.