Every size below is drawn at true pixel scale, so you can see what you would be giving up before you mark out a slot. Size is the one thing that cannot be changed after a space is created — advertisers build creative to fit it — so it is worth a minute now.
The one that works almost anywhere, and the one to start with.
For narrow sidebars where the standard rectangle will not fit.
The phone-width strip, and the only standard size built for it.
The tall column that stays beside the reader as they scroll.
Use the medium rectangle. It fits a sidebar and it fits inline in an article, it scales to a phone without leaving a gap, and it has enough height for an advertiser to say something rather than just show a name. Every other size on this page is a refinement of a layout you already understand.
Pair it with a mobile banner. A leaderboard needs 728 pixels of width and simply has nowhere to go on a phone, so a desktop-only slot on a mobile-first site is a slot that most of your readers never see.
You are not limited to these. A custom size fits an unusual template exactly, at the cost of advertisers having to make artwork specifically for you — which matters more for image ads than for text ones, since text creative is fitted to the box automatically.
Once you have picked one, create the ad space — or see what it might earn first.