Don't worry, the perfective/imperfective difference isn't easy for people who don't have this distinction in their native languages.
"Chcę napisać list" is expressing the will to write a letter and most logically you'd want to write a whole letter, not just half of it. To finish this action successfully.
"Chcę pisać list" focuses on the process, not the results, it suggests to me that maybe the letter will be so long I won't finish it in one sitting. Or it's "leave me alone, I just want to do some writing" (but still it's a bit strange if you specify it's a letter).