12"from the floor will not be visible above a sofa. Other options are up to the individual quirks of your old house's particular structure. Talk to your professional who is familiar with those quirks.

Return vent size and number are based upon calculations that your HVAC pro should be doing. They are a crucial part for the proper functioning of your heating/cooling system. This will most likely be looked at by the building inspector.

Graham

Old houses are drastically different than new builds and finding the appropriate contractors can be a difficult task. Anyone who plans to use standard sizes and placements will have a difficult time developing a good system in a vintage property. Returns can be different sizes and shapes and can be placed in the floor or on the wall, In general, individual rooms require individual returns, although bathrooms usually have supply lines without returns. Return sizing is different based on whether you are dealing with a system which is exclusively heating or also planned for central air conditioning.

There's a lot of thinking involved in laying out the HVAC system in any property and the layouts and framing techniques of 100+ years ago bring additional challenges.

Ductwork layouts are much more important than most homeowners recognize and mistakes are difficult to fix later. If you have doubts about the information you receive, keep working...a slight delay at this stage is better than a lifetime of uncomfortable rooms.