What normally drives these installs is the location of the waste line....unless you are moving that. Things are pretty much set. Normal position is 12" OC from the back wall. Round bowls have a slightly smaller "footprint"
KMS as always is right on target with this. You can only increase the space by choosing a round bowl rather then a oval type. The floor opening is pretty much set. You could purchase wall mounted toilet, but your really not going to gain that much more space and the cost of these toilets combined with what you would need to do to put them in you would be better off doing a complete construction renovation for the baths to gain the space you desire to have.
Valid points are made above but I would also say take a look at a on piece toilet as well. You can often times find a one piece elongated toilet that is the same size as a two piece round front. The elongated bowls are more comfortable and then you would also have the option for a comfort height toilet which is nice. The wall hung toilet will give you more room but there is more construction cost involved. The good point with the wall hung is that it is easier to clean because it doesn't sit on the floor at all and you have a range of height that you can mount the toilet so you aren't stuck with just standard height or comfort height. I would check with a plumber on the cost of a wall hung toilet to find out if it is an option for your bath. Check out the Kohler Santa Rosa one piece toilet it is a compact elongated toilet in comfort height and should extend out from the wall about 28".
The issue with a wall hung toilet is there needs to be some substantial "framing" behind that this mounts to as well as "non-traditional" plumbing locations for the waste line. Some plumbers will not tackle these as they lie outside of their comfort zone...ie: not their normal residential install.