They need to be at least the same size as the vent to which they are connected. Making them smaller would reduce air flow. Making them bigger does not help and is wasteful.

Yes- we have the opportunity to choose the vent and the register size

ok then, what size system?
what size house & size of rooms?
orientation of house, type of windows
amount of shading & size of overhang?
floors insulated? floors over a crawlspace?
basement?

there is quite a bit that goes into determining
what size registers & ducts that we are not
privy to.

your hvac company should be guiding you
as they have access to this info.

best of luck.

• 2800sq foot home
• Rooms vary in size. Downstairs open floor plan with largest room ~25x22, most bedrooms ~12x13
• House faces south, most windows do not have direct sun exposure.
• Overhang on eaves ~2.5-3ft.
• new insulation to be put in in crawlspace, attic area and walls per local building code. No basement, just crawlspace.
• Haven't purchased forced heating or air unit yet- want 2 zone system, may want one with variable speed setting.

HVAC sub didn't mention pros or cons to size, gave us option of 4x10 or 4x12

I think what he is talking about is the vent boot. Basically it is the piece he buys that connects the round duct to a rectangular opening. The 4X10 size should be fine.

Is the HVAC contractor going to do a load calculation?

I presume so - but he hasn't told us what that is. What is a load calculation?

I suppose I should say I live in a temperate climate - most of the older homes here don't have AC (ours didn't before), and we only use the heat intermittently for a few months

A load calculation is done by taking measurements of every room along with insulation values and orientation of the house and putting the data into a software program. This program will calculate the amount of heating and cooling is needed. This is used to properly size the HVAC equipment. Equipment that is too big is just as bad and sometimes worse than equipment that is too small.