Use CSS to hide it:
.google-dfp-ad-label { display: none; }
That label is a requirement on many sites. AdSense may integrate a notice in their ads, but other ad networks don’t and require sites to add their own labels.
As for ad sizes, that’s entirely up to you. Your ad network settings (ad units, inventory, creatives, etc.) determine which ad sizes you should choose. Don’t choose settings that don’t match any ad inventory you’ve defined otherwise the ad network will probably display a blank rectangle.