How do I change the default from cm to pixel in table properties?

How do I change the default from cm to pixel in table properties? It’s got to be really simple but I just can’t find it. I want to be able to state that a table is 900px not 10cm wide. Somebody please put me out of my misery.

Subject: RE: How do I change the default from cm to pixel in table properties?

Pixel-based measurements are not supported in tables.