it's a sample database intended to be used by the widest possible audience.
it is intended that you will edit the database to suit your needs, if you prefer to use decimal over float, by all means, edit the data type.
it is intended as a starting point for you to get up and running quickly.
if you want the useremail and password fields to be shorter, then you can edit them to be shorter.
why are they set that way? no real concrete reason except to give you a place to get started from.
as for the checkout wizard, you can apply your template to the pages after the wizard is finished.
re running the wizard is not suggested unless you first delete the existing checkout pages. the wizard is a page creation tool, not intended for editing.
