Look at instruments like the Dynon D10 and their engine monitor, they give you a full panel in 2 80mm instrument cut-outs. Add a Garmin GTN650 (with remote transponder) and even with back-ups, you can have a full IFR panel in a very small space.
OK the Dynons aren't TSO'd, but it show's what is possible, and that set-up would be IFR legal for a US experimental category aircraft. The Aspen EFIS is TSO'd and still pretty compact.