From your description "size can be defined and yet the window can be movable by clicking and dragging them" you want this for the desktop. My FormExDesktop library can do this (and more).
Updated FormExDestop and ControlsExDesktop libraries
Check out the demo app included - it shows a modal form.
You should check that forum for many other useful libraries - note that the latest versions of the libraries are sometimes reposted at the beginning of the thread and sometimes at the end so you may need need to read through the thread to find the current version.