I'm not happy about the way that circuit handles grounding, and I suspect there is a fair bit of inter-action between the volume controls.

If size is your main concern, an Alco Switch MTA 206PA is what you need. It's a miniature On-On-On double-pole, double-throw (DPDT) switch that does some useful things in the centre position. It's about the size of a sugar cube, but it will perform the same functions as the bulky leaf-spring type of switches that are more common.

My favourite supply house >Digi-Key don't carry Alco, but lists several equivalents from other marques.

Good luck!