“Common surnames” is a standard block. The #xxxx# variables for those were created to adopt the same options as the existing blocks.
Other statistical data elements such as #commonBirthPlaces# were only ever considered as just that, a list of statistical data.
However, I am currently working on the entire statistics code, to fix the problem of losing the Google Charts API (see: https://kiwitrees.net/forums/topic/statistics-charts/), and just adding an option to set the number of items in lists such as #commonBirthPlaces# is easy to add., So I will do that for the next release. Its use would be like #commonBirthPlaces:25#
Adding a scroll bar is not so easy. That works for blocks, like commonSurnames, because they already have the styling CSS in each theme to control their height and width. So a simple option to turn scrolling on or off is simple. To do the same for the lists like #commonBirthPlaces# would require at least additional options for the width and height of the containing element – as well as the “overflow: scroll” option – and I can imagine each use-case would want those to be different. The other solution would be to create “blocks” for all those as well, but there are just too many of them.
So I think, as presumably they are being embedded in an html block anyway, it would be easier for each site admin to wrap the #commonBirthPlaces# in whatever html element they want for it. For example: