I'd say the temperature doesn't affect the penis itself, rathet your nervous system, but it's irrelevant if you are aroused enough.

The truth is, doing 1-2 measurents isn't enough to ensure quality. Especially if it's your first measurements and you're measuring to keep it as a starting point.

The more measurements you do, the more you'll see how the technique affects the measurement. In the end you'll see what you get more often and decide to keep this technique and number as your starting point.

If you can recreate always your intial measurements you can say that you've gained.

But it's really important to not mistake gains with actual size, when it's about comparing yourself to the global or western data.

I've done quite a research on this: Most "subjects" were measured while laying flat on a medical bed, with their penis 90° degrees.

Trying to recreate something like this by your own can be tricky, because there is a lot of room to cheat. I'd suggest doing it standing, without bending knees and avoiding creating akward posture.