42" is Big Enough for a Small Yard Right?
Yes. But....
From a productivity standpoint, a 42 may be fine, but it is not uncommon for us to see customers who purchased 42" or smaller Zero Turns to later be seeking to purchase bigger tires for more traction because they are having trouble navigating a slope or are just tired of having to be extra careful to not tear up the yard. Having wider tires or larger diameter tires does help traction some by getting more tire on the ground.
But the bigger factor is the leverage created by getting the rear tires further apart from each other. It's similar to spreading your hands apart on a grocery cart handle. If you place your hands closer together, you don't have near as much leverage to wield the nose of the cart left and right. Like the grocery cart, the zero turn is steered from the rear.
For this reason a 48" or 50" zero turn with its wider stance will steer with substantially more ease than a 42". And a 34" has so much worse traction than a 42" that I jokingly tell customers a 34" starts to loose traction when it sees a slope in the distance, which is not much of an exaggeration. You have to be extra careful not to spin up the yard even on flat ground with a 34". Even if you don't have a ditch line or a steep area to deal with, the extra traction makes it easier and often quicker to navigate on flat ground.
An analogy would be driving your car on gravel vs pavement, you are able to steer and accelerate more abruptly on pavement than on gravel because you have more traction. On the other hand, don't forget to measure and make sure your new mower can go where it needs to go. Places like the storage shed door, the fence gate to the back yard, your trailer, etc.
Price
The price difference between a 42" and 50" of the same series mower is typically no more than $200. But the 50" it will handle noticeably better.
