So, EAC has been turned off for two days and although you feel better you clearly still feel DRY and air is still stale.
And yes it's had to imagine needing to add humidity in Fla.
I'll let the pros chime in exactly, but lets consider that you went from a 3.5 to 5 ton unit the fan speed has increased so you unit is moving more air. The return and supply ducts are pretty much unchanged, so if they leaked before, they leak now. As you mentioned you feel your home is leaky..
So if the unit is moving more air could infiltration go up decreasing humidity and causing this dryness???
But you say that air smells stale, which one would think is a sign of less infiltration.
How about this. If she was to open one window an inch or so and report back whether she felt air being pushed out or pulled in when system is running... Would this bit of info help.
Quite frankly, if your contractor doesn't have a simple hygrometer I'm not sure how much help he'll be with an IAQ (Indoor Air Quality) problem. Seriously consider buying you own at Radio Shack for $20. Target sometimes sells them even cheaper.