Big is used for things that are big relative to other things in comparison. A big man is what exactly? Probably a man taller than the national average of 5 ft 9 in... So a 6' 3" man is big... or is that just tall? Both can work here. Maybe you need a weight to go with that height to decide "big" or not. Just remember to use these adjectives with a sense of reasonableness (see other section). Same line of reasoning goes with small or little.
Long, short, thick, thin, deep, shallow, hot, cold, ... so many words to describe things. Most things in this world have common reference points for comparisons. So please pick and use descriptive words that fit into the realm of what you are talking about. I often talk about molecules that get really big - they are big-ass molecules. Realize that in the world of molecules, a big-ass one is like 20-50 nm in diameter or length (or even bigger). That is still so small you can't even see it with a microscope. But it is really big in comparison to other more common molecules (down around 2 nm or less). What is a small planet? My guess is it is still really big compared to you and me but maybe smaller than the earth. Know the realm in which you are comparing.
Use your own humanity to make the call on things - your senses. Touch something and it is hot? cold? cool? warm? Think about that. I ask the question (true or false actually) to my classes on the first day about is fire hot? Typically about 40-50% of the class says false, that fire is not hot. That is over thinking a pretty basic thing. Of course fire is hot. Fire is often depicted as an icon which means "hot". It is ridiculous to think otherwise. Also, ice is cold. There you go, two extremes that we humans can detect and make value judgements quickly. Now if you want to compare things as which is hotter, this or that? Just get a thermometer and measure - the big number wins for "hotter". Don't over think it.