Anything 12v will work - what you need to think about is AH. The standard was something in the neighborhood of 14 amp hours. You won't find anything small with that much power. However, you can find fairly small batteries in the realm of 8AH that will probably run your bike fine, at least during the daytime.
I ran my CB450 on a 2.6 AH battery that worked fine during the day (with minor wiring modifications to run at full charging capacity with the headlight off) - however, it barely ran at night. I went up to 6 AH and ran with no problems, though my bike is rated for 12.
I'd be surprised if 12AH didn't run your bike perfectly normally, even with a headlight on at night. You should consider the kind of riding you do. If you live in an area with heavy traffic (i.e. you're likely to be idling your bike at 1.5 for an hour at night), you might want to get something BIGGER than the stock 14AH battery. On the other hand, if you spend most of your time on the highway at 6K, you'd likely do fine with 8-10 AH.
As snorkelfork recently pointed out to me, AGM batteries are another option - twice as expensive, but smaller even than SLAs.