Performance,
There are a slew of saws that will allow a 24 inch b&c to be mounted. The question really boils down to what type of performance do you expect from the saw with the entire or close to the entire bar length buried? Completely different circumstance with a 24 inch b&c in a 30 inch log, vs. the same in a 12 inch cut. What chain do you plan on running is also a huge factor.
There is a big difference between felling and bucking something already on the ground. A saw used for felling must have the power behind it when you see things going not to your liking and need instant giddyup and go to get back on track, bucking not so much an issue. The smaller the hp the more critical the operating properties of the chain become.
Two rules of thumb I have heard, take them for what you will. Saw engine CC should be approximately 3 times the bar's length in inches, or the number of CCs should approximate the number of drive links of the chain. (3/8 pitch)
In the end it's your saw, you decide what is acceptable for your application.
Take Care