Hey everyone!
I’m new to the scripting region of Roblox and require the assistance of the community.
Let me explain what I need help with: I’ve always been interested in how to properly script a GUI that changes the size by dragging it(Something like Moon Animation Suite has[See example here]) but never knew on how to do so. I’m not asking for you to give me the script, just some tips on what pages I should look/read at to get a general idea.
Thanks!
Well, your first friend will likely be UserInputService.
It tells you where, when, and what was pressed at any input. You could use this to detect when the selection point of a object is pressed and released, and where the mouse is moved to or the finger is moved to (mobile).
Then, you would use the results the figure out how to change the .Size.X.Offset and .Size.Y.Offset accordingly.
If you haven’t scripted guis much before, you might want to look into Roblox’s Intro to GUIs to get a general idea of how guis and gui objects work.
For getting started with scripting: https://developer.roblox.com is a great resource to get started with scripting, with many articles and intuitive tutorials. However, there are also other sources you may find helpful such as youtube and the developer forum itself. There are hundreds if not thousands of posts on how to get started, so feel free to use the search bar and find some 
.
If you have any questions, feel free to DM me or continue the chat here or on another topic.
Good luck on your adventures though the wonderful world of Roblox developing!