Usage of 万 (wàn): ten thousands or just a great number? I noticed that very often the word 万 (wàn) occurs in poetry (e.g. poetry of the Tang dynasty). My dictionary reports its meaning as "ten thousands", but it looks like it's used in place of a generic "great number". A sentence I notice very frequently is "万里" (wàn lǐ): I feel like this is just a way to mean "a very long distance".

So, what is the proper meaning and usage of "万"? Is it correct to use it for "great", "enormous"? On the other side, to mean "huge", "great", is it correct to use "万", 10000, and not something else (maybe even numerically bigger), like, "one million", "one billion"?
You are correct, 万, 千 and 百 literally mean 10000, 1000 and 100. These words are often used as a figurative number for 'great', 'huge' or 'immense' 

For example: 

万人空巷 doesn't literally mean ten thousand people spilling out to the streets. 万人 means 'great number of people'  

万水千山 doesn't literally mean ten thousand bodies of water and one thousand mountains.  万水千山 means many, many mountains and bodies of water = "immense distance"

千锤百炼 doesn't literally mean hammer one thousand times and temper one hundred times. 千锤百炼 means "extendedly forged"

Other numbers like 六, 九, 十 are often used metaphorically for 'all, entire, ultimate' too.  

For example: 

六軍(whole army)，九阴九阳(extreme/ ultimate yin and extreme/ ultimate yang), 十全十美 (perfect)

万, 千 and 百 are not figurative number, when they are with actual numbers.  一万, 二千 and  三百 can only mean 10000, 2000 and 300