A bit of history first. The first machine made paper came from a steam powered mill in 1805!
So until then watercolour painters had to take any paper they found. The famous painter Thomas Gainsborough (1727-1788) bought paper that had been used to print a Bath tourist guide. Others used paper that was used to wrap packages. In the 19th century, paper was commonly stretched by artists, using water and then being laid totally flat on a board. Those days are gone and now the best art papers can last without significant loss of quality for over a century or more.
There are now so many paper manufacturers that it would be silly to list them all here and unfortunately there is no guide to which paper suits which artist. You will just have to try them out to you find your favourite kind. Watercolour paper is typically an almost pure white, sometimes a little yellow but nowadays you can buy any colour of paper you wish, it all depends on your imagination.
You shouldn’t save money on the quality of your paper, as it can make the difference in your painting. Poor quality fades very fast and is very difficult to work on, especially with water colours. You should also take care to buy pH neutral paper which will keep far longer and preserve your painting.
The basic weight of the paper is important and is worked out in grams per square meter. The basic range of watercolour papers is between 280gsm to 640gsm. If you want to paint wet on wet, your paper should have at least 300gsm otherwise the paper can not hold and will wave or rip.
The size of the paper is up to you and really only the amount of space you have is the limit. Most common is the full sheet which is 22”x 30”. I find buying single sheets of paper better then blocks as they tend to have subdued finishes.
The right quality of the paper is important but as history shows us, it’s the painting that counts.