Decoder outputs usually are rated for 100 mA (1/10 Amp), so aren't likely to be hurt by smaller resistances. At 14V, any resistor of 140 Ohms or more will limit current to 100 mA or less. Check the function current rating of the decoder you are using to confirm the 100 mA rating, or learn its actual rating and adjust the minimum resistor size accordingly (double the resistance halves the current, and so on.)
The eye is a lousy sensor of light intensity. It's not able to distinguish two levels that are similar. So, you need to change resistances quite a bit to change observed LED brightness much. Your various resistances were all low, making the LEDs very bright, and changes hard to distinguish.
100,000 is the same as 100k. The k is scientific notation for 1,000 times.
If you use a pot to find the resistance you need for the brightness you want, then I strongly recommend putting a 1.0k resistor (1,000 Ohm) in series with the pot (in the line from the power source to the pot) to limit the minimum resistance and maximum current to safe values. If the pot is used alone, then turning it to one end will present a near short circuit, allowing excessive current flow that probably will destroy the LED and the decoder function output transistor, if using with a decoder. Measure the resistance from the power source end across the resistor and pot to the load (LED) end for the fixed resistance you need.
At or above 1.0k Ohms, on 14V, the power dissipated in the resistor as heat is 196 mW (196 milliWatts/0.196 Watts.) A 1/4 Watt resistor will get pretty hot at 196 mW, being nearly 80% of its power rating.?
Above 2.5k Ohms, even a 1/8 Watt resistor is fine, and smaller than the 1/4W. The body size depends on the amount of heat the resistor has to dissipate into the surrounding still air, and how hot the materials it's constructed of can be without damage. Some power resistors can be operated at much higher temperatures without damage, but are hazards to human flesh and other typical electronic components.
If brightness is OK at 100k Ohms, then you are operating those LEDs at about 12/100th of one milliAmp, which is tiny. The resistor won't even get noticeably warm, nor will the Tortoise power supply load be significantly increased.
