manually resizable and overlay square in the screen with flutter

hi I'm trying to make a widget like the one in this image"(green one) with flutter , the main function is being resizable and overlaying. is there any idea to achieve this ? thanks

i found a pretty good package witch satisfy my needs ,at least for one part of my question. here is a link :https://github.com/zamaniafshar/Flutter-Resizable-Widget.git