Okay, just trying to understand because the original request wasn't clear.
If you have a long list of categories in your sidebar and you would like to have the list fully visible, two things can be the cause of why they aren't all "visible".
One: The height was set for the container displaying the category list. Usually this isn't "normal" but with some Themes and styles, it could be. Look in the
style.css for the style reference for the list of categories and check to see if there is a
height set. If there is, eliminate it or change it to something more appropriate.
Two: Using lists, the number of "children" or subcategories get pushed off the edge of the page. This is because the sidebar container is set to a specific width, and anything in it is confined to that width. There are several ways to "fix" this.
If you want to widen the sidebar, and there is room enough to do so, in the
style.css file in your Theme, look for a reference to
menu, or something similar. In that CSS selector, look for the width and change it to a larger number. NOW, the architectural structure of your web page will change. It's a domino effect. Change one primary container and the rest get shoved around. Check in your
content container and other containers within the structure of your page and subtract the numbers you added to the sidebar to help shuffle things back into place.
If the list is running off of the page because the indentions are too wide, looking something like a staircase moving off the page, then you can change the indentions so they are smaller, keeping the "steps" very narrow. Again, look in your
style.css file for the styles that contain your UL LI under
menu. Adjust the margins and padding accordingly.
This is just a short look at your options because I'm not sure if this is the right answer. Here are more detailed articles that may help you, or at least get you on the right track.
If the problem is that you want to control the number of categories and their subcategories showing on the list: