Is it possible to increase the reach of a monk unarmed strikes except by increasing in size?
There are a variety of feats and items that let you do this.
The Combat Patrol feat lets you increase your threatened area, which effectively gives you reach for AoOs.
Of course, there are some monk weapons that have reach - not your unarmed strike per se but can be used to deliver ki effects.
You can use the feat Lunge (Core Rulebook).
The only problem is that you need BAB 6 or higher.