How many BTU's do I need for a bathroom which is 12' x 6' with 9' ceilings? I am looking at kickspace heaters. Any brands that are very good?

You need the smallest one you can find. You only have 72 sq ft to heat. All brands the same.

hvac1000 is correct. Not sure what area you live in so cannot recommend any BTU ratings. But here in VT you would need about 1500 Btu's for that size bathroom. Something a inferred lamp would heat if you did not want to pay the bucks to put in a kickspace. Consider a towel warmer!