If it has more than one anchor set, it might be locked to the size of its parent. If you run
:ClearAllPoints(), it'll wipe them out. You'll then need to redefine at least one anchor with
:SetPoint() so the game knows where to render it on screen.
For example:
Lua Code:
NamePlate1.UnitFrame.healthBar.background:ClearAllPoints();
NamePlate1.UnitFrame.healthBar.background:SetPoint("CENTER");
NamePlate1.UnitFrame.healthBar.background:SetSize(100,6);