Every time I develop a new form that includes a textarea I have the following dilemma when I need give its dimensions:
Go through CSS path or use their attributes col and rows?
What pros and cons can I found using both paths?
What semantic information gives using these attributes?
How people usually does it?
According to the w3c, cols and rows are both required attributes for textareas. Rows and Cols are the number of characters that are going to fit in the textarea rather than pixes or some other potentially arbitrary value. Go with the rows/cols.
I recommend to use both. Rows and cols are requrired and useful if the client does not support CSS. But as a designer I overwrite them to get exactly the size I wish (not to mention you don't do this inline via <textarea style="width: 300px; height: 150px;>, do it in an external stylesheet).
The answer is "yes". That is, you should use both. Without rows and cols (and there are default values even if you don't use them explicitly) the textarea is unusably small if CSS is disabled or overriden by a user stylesheet. Always keep accessibility concerns in mind. That being said, if your stylesheet is allowed to control the appearance of the textarea, you will generally wind up with something that looks a whole lot better, fits into the overall page design well, and that can resize to keep up with user input (within the limits of good taste, of course).
The size of a textarea can be specified by the cols and rows attributes, or even better; through CSS' height and width properties. The cols attribute is supported in all major browsers. One main difference is that is a container tag: it has a start tag ().