CSS is a bit complicated, everything depends on the specific context, so I can’t give you anything too specific on this.
I have an article that will provide you with some very useful tools for editing CSS rules: Simple CSS Techniques for WordPress
The issue with the borders is most likely that you are not using a selector that overrides whatever is adding the borders in the first place. Using the developer tools (as described in the article) can tell you what rule is applying the borders, which will help you write a selector that will override it.
Changing widths can be considerably more complex, since it’s very likely the form is nested in several other containers which may be affecting the width. Figuring out which element to target can be tricky, and often table elements can’t be made less wide by simply changing the width, you usually have to make the contents less wide because the table will tend to expand to contain whatever is in them.
