Haha, we understand that it's one of the most requested feature :)
Unfortunately, implementing that on a 2D-based generator like this is quite problematic. The solution that we'll be using in the future is providing a set of pre-defined sizes, such as small, medium, large and so on.
A bit clunkier than a slider, perhaps, but it'll be a start until we can implement something better.
Thanks for your support!