Look into the UMG system. This should definitely be viable, but we don’t have any documentation on resizable windows at present. If you would like more information on UMG, however, check these out:
We also have several tutorials for UMG on the wiki. Though they aren’t directly related to what you want to do, perhaps you could find some information that would be useful to you:
I look forward to seeing what you come up with in the future!