I assume when you mean smaller you mean one with a remote reservoir.
The 919 M/C is 14mm
The 929 and 954 are 19mm and RC M/C's are either 17mm or 19mm.
Putting on a 17 or 19 mm M/C would give you a very hard lever and very little feel.
You could change the calipers to the appropriate ones and have a balanced system. The calipers are interchangeable.
In addition the M/C and calipers from a F4i work.
There are a number of threads on this in the search section.
Just a gentle warning, I don't think you are going to save as much room as you think you are. You might be able to eek out a little more space but I don't think its going to be a lot. I guess every little bit can help though.
88 Blue Hawk (NT650) Project Pics - http://tinyurl.com/clw8h3q
2000 DL650 - Being ridden but make me an offer if interested...
2002 DL1000 V-Strom