One ad size per slot, or several?

One ad size per slot, or several?

An ad slot is the box on your page. You can let it accept one size, or several sizes at once.

— Single size (300x250): predictable layout, no surprises, but fewer buyers can bid.
— Multi-size (300x250 plus 336x280 plus 300x600): more buyers compete because more of them have a matching ad, but your layout can jump if a tall ad lands where a short one was expected.

Tiny example: allowing 300x600 alongside 300x250 in a sidebar invites richer bids, but reserve the taller height so the page doesn't shift.

1. Add sizes that physically fit.
2. Reserve space for the tallest.

In plain English: more sizes mean more bidders, but lock the space so nothing jumps.