All I owned was a 6" jointer for about 30 years
The 6" was an old Craftsman with a fixed outfeed table, about the lowest cost version available. When you say making furniture, there a lot of pieces under 36" and less than 6" wide for which a 6" would be fine. Wider boards than 6" are not used much except in table tops and then they may be up to 8 ft long, no match for a 6" jointer. So, the size of the jointer is relative to the size of the project and the boards you need for it. I owned a beautiful 8" Grizzly for a short while, thinking I was going to go "big", but that never happened. I now own a 13" jointer/planer which works for everything I do.
The primary function of the jointer is to straighten and flatten the face of the boards. While the jointer is also used to straighten the edges of a board, they can also be straightened on a table saw with a straight line jig. I use this metrhod with great success. So, the width of the face is the determining factor. There is no other woodworking machine that will do this operation as easily and as quickly as the jointer.
The longer the boards, the long the jointer beds should be. You can make matching height supports if push comes to shove and I have done that with good success on the rare occasions I needed it.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)