I started on a 750 (and still ride it). You can do it. The little bikes are way more forgiving though, and you will have mistakes. The smaller bikes are easier to maneuver a slow speed.
Also, you can buy a used 250 or 500, ride it for a season, and sell it with no significant loss at the start of the next season. Little bikes hold their market value really well. At your size, a small bike like the Ninja or a Rebel will have enough power to move you quickly. A friend of mine who is a lot bigger than you takes his v-star 250 on the highway without any trouble, and absolutely loves it when he’s driving downtown. Also, you’ll love the ability to park pretty much anywhere. If it can park a bicycle, you can probably put a 250 in it.