I’d recommend starting with a metric in the 600-750cc range. Can only speak for myself but I think if I started with anything smaller as an adult, I’d get bored too quick and just jump to the big dogs. 600 is big enough that you can get some limited experience at highway speeds but not anything too heavy that you’ll dump for beginner mistakes. And if you do, it’s not something you spent a lot on and could probably sell it for what you paid. Stuff like stopping without having your handlebars square will through you off balance. On a smaller bike you’ll be able to catch your mistake and hold the bike up. A bigger bike might just fall over. Slow speed maneuvering is a lot harder that than it looks. You’ll screw up a lot more small stuff than you’re currently anticipating. Definitely see if you enjoy riding before you make an expensive purchase. I hope you have a good time figuring everything out