The input attenuator is designed as a separate board, since some people will not like the PGA4311. It would add a lot of hassle to add it to the board (due to layout) - so I think a separate board is ok (for me).
If you look for a stepped attenuator check Twisted Pear's Joshua Tree attenuator.
The height of the boards is about 2.2 inch. The width of the input selector is roughly 4.2 inch.
The maximum component height is defined by the plugs & capacitors. I think something like 1cm (~0,4 inch) - perhaps a bit more. You should be able to use horizontal connectors if you have more space horizontally than vertically.
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