wp ads manager

real noob question but seriously can't find the setting to do it. Got this message Ad unit is too small, vi stories is not working on your site. The minimum vi stories unit size is 336x280px. Please insert the tag into a bigger unit. But can't work out how to change the size of ads? Any help...

I have been trying to work around this for some time now with no success. Ads blocker turned off, yet ads not displayed. Most of my site users visits through Opere browser. Any one with useful information on this?

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