Avoid listing the same word multiple times in a row since some engines may penalize for this. Instead, use the keyword multiple times, but separate them by other words in your text. Use the longer form and the plural form of a keyword when possible. For example, if you use marketing in your Title tag, a search on marketing or market will yield a match on most engines. In these cases you'll want to try and use both forms of the word.
Longer titles are generally better than shorter ones.
However, shorter ones can be used if you need to better emphasize a keyword that you're having trouble ranking well with, or if that engine appears to favor pages with shorter titles. A common mistake in defining keywords is to compile a long list of single words that are somehow related to the content of your site. In practice, few people search for single words, because most people know that single word searches typically yield tens of millions of hits. It's far better to define a small number of keyword phrases that people actually use to search. Keywords should be repeated as little as possible.