I bought a cheapo battery for my old 1.8 Sierra a few days ago. It was only £24 and to my surprise only 20Ah Had "37 amps" in brackets which I presume is the cranking current. The old battery was rated at 44Ah and the even older one in the garage was the same. It surprised me a 20Ah is considered enough to be reliable, but time will tell. Its the same size as the old battery but a little lighter. The battery will no doubt last the life of the car now as I probably won't be using the car for more than another year.
Graham