Dynamic regions are regions that fit an unlimited number of blocks and will set the block width by dividing the space up equally. This means that when you have on block in a dynamic region it will take up 100% of the available space, when you have 2 blocks they take up 50% each and when you have 3 blocks, each block will take up one third of the space.

Technically you can add an unlimited number of block in these regions but realistically you are limited to the amount of screenspace your users have. Though If you're using a theme for a presentation on your Full-HD projector, you can add twenty blocks to a dynamic region and they will look just fine. This system is often used in the regions called "preblocks", "postblocks" and "Big Footer".

Another (optional) feature of dynamic regions is that a all blocks are automatically set to equal the height of the tallest blocks, to ensure your website looks tidy.

If you configure your website in such a way that different pages have a different number of blocks, you do not need to worry about the widths being wrong. Block widths in dynamic regions are calculated separately for each page.