Which Vector3 setting to use when trying to cover the entire map with a baseplate?

Unfortunately, there is a limit to the max X, Y and Z scale: 2048 studs. It seems you have reached this limit. However, what you can do is reference or create another part, size it to the rest of the map, and position it accordingly. Here is one example:

local restofmap = Instance.new("Part")
for i,v in pairs(workspace.SpecialParts:GetChildren()) do
    if v:IsA("BasePart") then
        v.Size = Vector3.new(10,10,10) 
    v.Position = Vector3.new(X,Y,Z)
    restofmap.Size = Vector3.new(X,Y,Z)  -- Size of the rest of the map
    restofmap.Position = Vector3.new(X,Y,Z) -- Position of the rest of the map
    restofmap.Parent = workspace.SpecialParts -- Placing the new part into "SpecialParts"
    end
end

You can do this in other ways, but in most cases, this is the most effective. If there are any errors or something is wrong with my script, be sure to let me know :)