I had to change some fontobjects you created to use GameFontNormal so I'm not sure about the actual EditBox size but that's another thing this might hightlight.
In your PLAYER_LOGIN you can use.
Code:
NewbDevBoxInterfaceFrame.ScaleTextBox:SetNumber(NewbDevBoxOptionsTable.ButtonFrameScale * 100)
NewbDevBoxInterfaceFrame.ScaleTextBox:SetCursorPosition(1)
NewbDevBoxInterfaceFrame.OpacityTextBox:SetNumber(NewbDevBoxOptionsTable.ButtonFrameOpacity * 100)
NewbDevBoxInterfaceFrame.OpacityTextBox:SetCursorPosition(1)
Which should show some of the numbers at least.