So 4-6 boxes is indeed the goal, you just want to make sure you're using you're utilizing your page space as well.

Take a look at your submission and look at box 241, I'd say around this size is what you should aim for. Notice how the other 3 boxes on the page are noticeably smaller and there's also tons of empty space left on the page as well. Drawing larger doesn't necessarily mean that you should draw so large that they're all overlapping one another but you could push box 241 down and to the right a bit more and draw the rest of the boxes larger while utilizing the space better overall. You could fit 6 boxes (or 4 if some of them were longer/wider/thinner etc.) the size of 241 on this page.

In contrast to this if we look at the first few pages in the submission, there's often 50%+ of the page not being used. Or you can look at the page with box 65 on it and the top 3 boxes are quite small and the top 2/3 of the page is barely used.

While how you use your page isn't necessarily a sign of the quality of your work, drawing large is beneficial, and using your pages well not only saves you material in the long run but can also help you slow down and be more mindful of the process. Remember that mindful practice often helps people retain what they're practicing better than just going with the flow of things.

Hopefully that helps clarify things a bit.