I suggest showing the size of each planet structure that you currently have built on the structures page of the planet, and in addition once you click on the "build structures" button I suggest showing the used/remaining space of the planet somewhere on that page.
This would help streamline the building process as it seems a little cumbersome now, clicking back and forth between tabs to check the size of things or to purchase materials for construction.

Well it already tells you the total ammount of space is used up by buildings but I like the 2nd one, that would save some loading time.

Yes but it doesn't tell you the space used by each individual building. I find that I'm toggling back and forth between the structure and construct tabs when I'm trying to figure out how much space I need to upgrade my facilities. I think this is the area of the game that could use a little more attention to smooth out game play. I know it's not as glamorus as battles or ship improvements but I find this area of game play the most cumbersome for me. The rest of the game is actually pretty smooth for me....good job Dan!

Oh I get what you mean now, you want to know what structures to get rid of in order to place new ones, yes that makes sense. But I keep all that info in my head.

I wish I had that ability Robert

How about this idea: it can give you an upgrade button that allows you to destroy and build a new building at the same time, with the extra benifit that you can't accidentally destroy a building without the resources to build the next level.