Login to Ad Manager. Scroll down to the bottom of the page and click on:

Click Zone Configuration on the next page:

Make sure you're in the right web property. If you're not, just click on the drop-down menu and select the property you wish to work on:

To edit a position, go to that position and click the pencil icon:

Click the check box next to the position you created to activate it or uncheck it to deactivate it:

Now you can add or remove dimensions and set/change limits. To add a size, select it from the Dimensions drop down then click the -> to add it.

The dimensions you added will appear here:

To delete a dimension, click the trash can icon next to it:

Dimensions are auto-saved when the arrow icon is clicked. To modify the  Impression Limits or the limit of time-based bulk ads available for this position by month and year.