If you're staying small, make your life easier and give yourself the best opportunity to make some money by only allowing the most common types of ads and ad sizes on your site. The more obscure the size, the fewer ads that will be available. If you're not doing direct sales to marketers, you might find that ad networks like Google AdSense are limited in the number of ads they have to place.
The easiest ads to get are text-based ads. These usually consist simply of a fairly short title, a couple lines of copy, and a link to site being advertised. They're not particularly attractive, but you're pretty much always guaranteed.
If you want the ads on your site to be more visually stimulating and you want to showcase display ads (in place of or in addition to) text-based ads, the most common sizes are as follows (measured in pixels):
Although there are numerous other sizes for ads that are also popular with both advertisers and site owners (small 120×90 ads, called “buttons,” are starting to become more common), if you stick with these three most oft-used ad types and sizes, you'll not only be able to blend your content with your advertising effectively, but you'll be reaching into the largest pool of advertising available.
As with anything else, the bigger you get, the more complex things become. If you want to make a career out of selling ads online, you're basically making the decision to start your own media business. There's a lot more to know about it than just how online ads work; you'll need to know how to start, grow, and manage your own business as well. In this section, I'll review some of the most important pieces of info you'll need to know with respect to generating revenue from online ads, but will stop short of giving general “how to grow a business” info such as hiring employees, dealing with taxes, etc. For that, you can check out another book I wrote awhile back, titled 201 Tips to Start and Build Your Own Business.