What battery should be in the XF-125GY ?

I have a 12v 3ah battery, which needs replacement, one dealer says it should be a 12v 9ah, but the Portugese Sundown manual says 12v 6ah, which if any are correct , what capacity comes with these bikes originally, or what is best fit given replacement is necessary ?

Also, in which way should the battery be fitted in the battery tray, crossways across the frame, or vertically ?