The smaller the slide and barrel, the less reliable, at least for the 1911 design. 4 1/4 inches is the shortest barrel that will work with a barrel bushing. 4 inch and shorter guns work better with bull barrels and no bushings, although Colt's Officers ACP used a very thin bushing and a bull barrel and it seemed to be pretty reliable, at least until the bushing broke.
The ultra short 1911's can be made reliable, usually, but it may take a lot of work. A good compromise between reliability and concealability is a Commander slide mated to an Officers ACP size frame. Colt made these for a very short period of time and called them the CCO.