I'm trying to change the size of all of the bricks in a model. Why doesn't it work?

You should start by fixing your tabbing so that it is easy to read your code:

a = game.Workspace.Map:GetChildren()

script.Parent.Touched:connect(function()
    if script.Parent.Parent.Touched.Value == 0 then
        script.Parent.Parent.Touched.Value = 1
        wait(25)
        for index,v in pairs(a) do
            for i = 1,625 do
                local s = math.random(2,6)  
                v.Size = Vector3.new(4,s,4)
                wait(.005)
            end
        end
    end
end)

I'm not sure why you are using a IntValue when you could just be using a variable:

numberTouched = 0

...

numberTouched = 1

The minimum amount of time that wait is allowed to wait is about .034 seconds, which is 7 times slower than the amount you are trying to wait.

Your code attempts to resize each part individually 625 times, then moves onto the next part. Each part would take about a third of a minute, then.

If there are other objects in Map other than parts (e.g., Scripts, Models), though, you'll have a problem -- because they don't have a Size property.

You should check that v:IsA("BasePart") to ensure that v is in fact something that can be resized.

You should be using better variable names. a is a bad name for the list of objects in a map. Just say map or mapChildren or some other useful variable name.