If you attempt to create a cuboid to use exactly as many blocks as the projector claims it supports, a corner is missing every time, which means the block limit is one too low.

I decided to try looking through the source on github to find out why this happens, and I discovered it only happens if you use a shape card. It happens because PlaceBlockIfPossible, called from ShapeCardItem.composeFormula, limits the number of returned blocks to maxSize - 1, and shieldTEBase.composeShield passes it supportedBlocks directly, assuming composeFormula limits to maxSize.
I'm not sure why they disagree, you'll have to decide which limit makes more sense

edit: This comment got damaged somehow, fixed

Fixed next release