That would be a better benefit to the learning curve. Not only do you need to be comfortable ON the bike, flat footing, handle bar room for slow speed turns etc. But you also need to be comfortable with the power and power delivery of your first bike.
It’s not impossible to learn on the bigger bikes, can make things on the bad side of interesting though and have more potential to spooking you to riding all together. As many on here have said…the 250’s hold their resale value very well, get atleast a minimum of a riding season under your belt to get the coordination, awareness, and overall skills you will need to prepare yourself for the bigger bikes. Its easier to adjust an acquired skill to suit the ride difference then it is to learn through an intimidated mindset.
The 250 will give you the muscle memory, not only for good shifting and turning but also to get your body adjusted to how the bike reacts under certain conditions such as a loose back end, hard front braking, counter balancing in parkinglots the timing of a mild speed turn and the time consumption that increases as your speed does in response.