[Fizzlemizz]

You could do something like:

local function SizeName(self) 
    local font, size = self.name:GetFont() 
    self.name:SetFont(font, size-2) 
end 
SizeName(PlayerFrame) 
SizeName(TargetFrame) 
for i=1, 4 do 
    SizeName(_G["PartyMemberFrame"..i]) 
end

Add additional unit frames to size. Adjust the size to your liking.

You can play with the naming and turn it into a macro or copy/paste to addon.bool.no to create/download as a small addon that will run when you log in.