Every state has their own homeschool law that governs what records have to be maintained and/or submitted each year. Virginia is a very light state, all that is required is to submit an annual Notice of Intent (NOI) and then either a) the results of a nationally normed standardized test or b) a letter from a qualified evaluator. If you are going to work with an evaluator, you should plan on saving samples of student work from throughout the school year in at least math and language arts. You should easily be able to fit this minimal amount of recordkeeping in a 1" binder - maybe less. This is all that is technically required in the state of Virginia. Until you start having courses that would count for high school credit this is adequate.
Once you have a student with classes that would normally be considered high school classes, it is important to know that the public school system is not required to accept homeschool courses for high school credit. They might. They might not. Having thorough records for each subject, including a textbook list and work samples, gives a starting point for conversations about credit if it ever becomes necessary. It is also important to document in this way if you have a student athlete who wants to be considered in an NCAA sport (NCAA regs are intense, folks. Read their website and start prepping as soon as practical or you will be hurting at college application time.)
I plan to use an evaluator to meet the state requirement until 10th grade, when the NMSQT/PSAT will be available for Emma. Our evaluator writes on the state required math and language arts but also enjoys seeing what we worked on in other subjects. I keep a 3" binder for each year, with tab dividers for each subject. I put all completed work in the binder as we move through the year. At the end of the year, I winnow this down to what I will present to the evaluator - the grade sheet, tests, and major projects in each subject. I also save my NOI letter, any acknowledgement from the school superintendent, all of my assignment sheets, and a copy of the year end evaluation letter.
Be sure you know what the requirements are for your state! What works for me in Virginia may not work for you in Maryland or New York or Massachusetts.