When patch 3.3.2 was released, several frames started to be affected by the UI Scale (Esc -> Options -> Advanced). The most noticeable frames (in my opinion) happened to be the loot roll windows, prompting me to write the addon Roll Window Fix. WoWInterface user elsinnombre asked in the comments section of that addon for another which would fix the RDF "Your group is ready!" popup. I am happy to say, a year and five months later, that I have finally written an addon to do that, which also works for any other user specified frame.
To have a frame always be "full size," you must tell Frame Scale Fix to scale that frame. This is done by typing /framescalefix NameOfFrame or /fsf NameOfFrame. Assuming the given name is actually a frame, it will immediately be scaled to the proper size. Any time the frame is shown, it will also be checked to ensure it remains at that size. Furthermore, Frame Scale Fix keeps track of which frames should be scaled across game sessions, so you will not have to enter each frame every time you log in.
To remove a frame from Frame Scale Fix's list, simply use /framescalefix NameOfFrame again. Note, however, that Frame Scale Fix's scaling is not instantly removed and may require a reload of the UI (/reload) to properly go back to normal.
Frame Scale Fix's watch list is empty by default. The commands for adding the loot roll windows (if you don't have the Roll Window Fix addon) and the RDF popup are provided below.