Does the size of the unit call for different sizes in the ductwork when the unit size increases/decreases by around 2 tons or so?
99.9999% of the time yes.
There is the odd chance that the original duct installed was way too big and will work fine with the larger system, but thats very very rare.
Only way to know is have someone who knows thier stuff do a load calc on your house, then check out the duct to make sure it's sized properly (either using old or putting in new).
Most definitely, assuming they were right to start with. Run a Manual D and see where you stand.
"That's good enough..." usually isn't.
Yes size does matter. (makes you want to snicker does'nt it)
Ask your contractor if he owns a ductalator, in fact 2 ductalators - 1 for ridged, one for flex.
90% of all products are designed for a max static pressure of .5" WC. If you exceed the max - you'll have a host of undesireable side effects IE:
Noise, Evap. H2O blowoff, premature compressor failure, premature HX failure, loss of effiecancy, poor heat, poor cool, voided warranty, just to name a few.