Hello...I have some questions. We recently bought an older home (1910) and are thinking of finishing a room in the basement to use as a living space. The home used to have a gravity furnace but was replaced about 5 years ago with a forced air unit. There is plenty of headspace in the basement except for the huge cold air returns that still remain and are in use with the current F.A. furnace. The house is about 1300 sq ft. and only has two cold air returns which are on the main level (2 story home). The c.a. returns are huge, about 12x14. There are about 7 heat registers in the house, going to all floors and one in each room (about) but those are about 7 in round. I am wondering if I can make these c.a. returns smaller to fit into the joists in the ceiling to leave more headroom in the basement, that and they are leaky and I feel like they are not that efficient to begin with. I want to do this myself but am wondering if I would be starving my furnace of cold air, since there are only two returns to begin with (along with one directly on the side of the furnace in the basement). Both of the c.a. returns we have are that large for about 8 feet of ductwork but then lead into a 14x8 duct that sits in the joist of the ceiling, so would it even matter if I make the rest of the duct smaller to fit in to the joists?
Also...will we have a hot upper floor in the summer do to the fact that we don't have any cold air returns on the 2nd floor? How will air be ciculated? It is nessicary to try to add a cold air return on the top floor (because it would be mearly impossible with our plaster walls and considering the layout of the house...and, there is not a laundry shoot to take advantage of).
Thanks for any help you can offer...also, would we even need a cold air return in the basement? The finished room would be about 13x13, plus a small 1/2 bathroom on the side of it.
Thanks!

You should have return air grilles in every area that receives supply air...not necessarily every room, but every area. The size of the return air ducts is governed by the volume of air carried within them, measured in Cubic Feet per Minute (CFM), and by the velocity of the air as it moves through the duct, measured in Feet per Minute, and to a small extent by the length of the duct run back to the furnace.

Only if we knew all three of those numbers can we comment on resizing the ductwork. Your best path might be to consult a reputable HVAC contractor, who can measure or calculate volumes and velocities and make recommendations for you.