You need one scale before the Pixelate node, and one after it.
In my nodes, I scale the image down to one tenth of its size (the first scale node), pixelate it (the Pixelate node), then scale it back up to its original size, with the second Scale node.
The relation of the scale values is the mathematical inverse. Say you scaled it down by .5, that is just multiplying it by .5, or dividing by 2. So to scale it back up to the same size you need to multiply it by 2.
Here is an example table of the values. The first scale value and then the second scale value.
Note that the strongest pixelated effect is at the top of the table, and the weakest is at the bottom.