So, I am trying to make a block increase size when you press "g". What's wrong with it?

Here is the code. I use a localscript and just put it in workspace.

s = game.Players.LocalPlayer:GetMouse()



s.KeyDown:connect(function(key)
	key = key:lower()
	if key == "g" then
	local	f = Instance.new("Part", workspace)
	f.Anchored = true
	f.CanCollide = false

			for i = 1, 500, 0.1 do
   				 wait(0.1)
    			f.Size = f.Size + Vector3.new(1, 1, 1)			
	end
	end
end)

Close, first off there are some changes that should be made:

plr = game.Players.LocalPlayer
chr = plr.Character
s = plr:GetMouse();

s.KeyDown:connect(function(key)
    key = key:lower()
    if key == "g" then
    local  f = Instance.new("Part", workspace)
    f.Anchored = true
    f.CanCollide = false
    f.Position = chr.Head.Position + Vector3.new(2,-2,0) --just so it dosent go to (0,0,0)
 spawn(function() --Create a new thread to handle part seperately
            for i = 1, 500, 0.1 do
                 wait(0.1)
                f.Size = f.Size + Vector3.new(1, 1, 1)          
    end
end)
    end
end)

However, the method you are using is highly ineffecient according to the Roblox Wiki. It suggests using UserInputService or ContextActionService.

Let's take a look using ContextActionService:

context = game:GetService("ContextActionService")

function newPart()
 spawn(function() --Create a new thread to handle part seperately
            for i = 1, 500, 0.1 do
                 wait(0.1)
                f.Size = f.Size + Vector3.new(1, 1, 1)          
    end
end)
end

context:BindActionToInputTypes(
actionName = "newBrick",
functionToBind = newPart,
false,
Enum.KeyCode.G
)

For more information on ContextActionService, visit http://wiki.roblox.com/index.php?title=ContextActionService_tutorial

Above Code Is Untested