Bearcat F8F, don’t forget the adage that ‘form follows function’.
IMHO a small display (size) would not be suitable for a jet; a critical issue is the display resolution.
Whereas an instrument display can be created on a very small helmet mounted reflector, it is the angular resolution which enables usability. Thus the display size-to-viewing distance is an important relationship.
Also consider what has to be displayed. A small jet will probably have a much greater speed range than a glider, but still require the same speed resolution for accurate flight, thus the length of the speed scale should be greater, requiring a large dial circumference. Swept area is an important parameter for detecting change and rate of change; this could bias you to avoid tape speed displays as they could require additional supporting information such as speed trend vectors. The same would apply to altitude, but less so to VS as this is already a rate parameter.
Attitude flight might be more critical in the jet; consider the attitude-speed change relationship. This might warrant a larger attitude display to maintain resolution.
Navigation in a jet will be at higher speeds (both IAS and TAS), this requires a greater look-ahead capability, and this needs a larger display if resolution is not to be sacrificed. Don’t forget the clock.
Radio navigation is required for IFR operations, these require control-panel space; see cockpit remarks below.
Keep the design simple. Complex systems (design) can be acceptable, providing they avoid complexity (enabling simple use) by being easy to understand and providing timely relevant ‘information’, not just data.
The cockpit should be one of the first considerations as many airframe and system aspects could be dictated by cockpit size / space. Aircraft control column/stick lever arm and displacement could limit the control forces and thus change control surface size or system mechanical advantage – differences between gliders and jets - speed squared.
Panel space for additional navigational systems, radios, and engine system displays.
Don’t forget about night flying – IFR, lighting position and control.
Glareshield size/position must enable a good over-the-nose view and act as a sunshade. Consider sun reflection issues on instrument displays.
You may not be able to fix everything, but at least identify and consider critical items.
Good luck – ‘ s/he who makes no mistakes, makes nothing ’.