Before painting on a large canvas try a small version first. It's always good to start with sketches but there are benefits from planning with paint. You might create a small painting that is frame worthy. You will have worked out the composition, values and colors before you ever start the big project. Painting a large painting from a small one requires the addition of more information and it is easier to do that if you have already worked out the rest.
Recently I have been pushing myself to paint larger for the experience. I have been reluctant because I am not represented by a gallery. Selling large paintings online is more difficult than selling small ones and I don't want to end up with a storage unit full of paintings. That is the practical side but there is another reason. Whenever I see a small and large version of the same scene, I almost always prefer the small one no matter how well the large one is executed. Smaller paintings are more simplified because they have to be (unless you are painting with mouse hairs). With a small canvas it is easier to get a favorable brush to canvas ratio- the bigger the brush the better but you can only go so big with a huge canvas. Oversized brushes and large palette knives are available but are cumbersome and I find enormous brush strokes distracting. In theory small paintings can be painted more quickly and thus tend to have a spontaneous feel. Plein air paintings have to be spontaneous because of time limitations.
So how small and how big? This is entirely personal. Start with a small size that is comfortable. Paint as large as you want to paint or is practical for you. Consider your studio space, storage space and your easel. I once painted on a canvas larger than the easel I had at the time could support, so I had to lean it against the wall and sit on the floor to paint. I do not recommend this. As I said above, I have been pushing myself to paint larger in spite of my preference for smaller paintings. The more I do it the better I will become. I am trying not to worry about storage (up to a point) as long as I am growing as a painter.
Here is a recent project. My comfort zone is 8" x 10" to 16" x 20". The small painting is 6" x 8" and the large one is 24" x 36". Note that the proportions are different so I had to add more of the original photo to fill the width of the canvas. It would have been easier to keep the proportions the same. This was a big jump. Some artists paint an intermediate size before going really big. Also keep in mind that the more you paint a subject the better you will know it, even if you paint it the same size every time.
I encourage you (and myself) to get out of your size "comfort zone" while keeping the practical aspects in mind.
Thanks for reading!