Hi, Serhot. The Halfbike has a character of its own and comes with a learning curve that takes some time and practice. Below are links to two independent reviews of the Halfbike 2. I think they give good views to how most people approach and perceive the experience. Have a look if you're curious and in case you haven't already.
And there's also Halfbike 3, our latest model. Its proportions are a bit different, it's smaller, more intuitive, has 4 gears, a front brake and many other small mechanical tweaks. It also comes with an all aluminium handle and more colour options. When you have some free time on your hands, you can catch up on all info about the new model here >>
To sum things up - yes, both Halfbike 2 and Halfbike 3 have different sizes; most of the small parts (screws, nuts, pedals etc.) are standard and widely available, as well as the tires - most bike or skate shops would have them. In the unlikely event that you need a custom made part we could send a replacement. As to the learning curve - it's challenging but most enthusiasts are able to get the basics and ride in a straight line in under 10-15 minutes. From then on riding starts gradually to feel like less of a struggle and more like fun. :)
As this message will grow immensely, feel free to contact us at firstname.lastname@example.org. We'd be happy to share more details or answer other questions that come to mind :)