@peregrine23:

If you don't mind modding your ImprovementDefs.xml file, you could do that by lowering the LandPercentageMin property.

Example to allow multiple tiles with quality between "Habitat Improvement Plant" and "Resequencing Station" to be terraformed.

Change:
<ListIcon>BiosphereManipulator_Icon.png</ListIcon>
<ImprovementType>Terraforming</ImprovementType>
<IsColonyUnique>true</IsColonyUnique>
<LandPercentageMin>0.35</LandPercentageMin>

<!-- Stats -->

To:
<ListIcon>BiosphereManipulator_Icon.png</ListIcon>
<ImprovementType>Terraforming</ImprovementType>
<LandPercentageMin>0.20</LandPercentageMin>

<!-- Stats -->

I don't see a way (in the xml files) to limit the number tiles to a fixed number, like it was in GalCiv2. I would like to see if Stardock could introduce a property that either alters the number of tiles an improvement works on or the number of times an improvement can be build.