The default is 10k. If this is a new installation, that is the answer. In the very rare occasion that this is replacing a 12 year old or older Energy stat (square OJ Microline from before 2010 with red and green sensor wires), choose 12k. If this is a replacement for the other 99% of the other thermostats we sold (Honeywell TH115 and TH111 thermostats), 10 k is the value.
Always start with the default 10k and change later if needed.