The two smaller boxes have the same collision box size, the larger box is the original size for the left small box.
Colbox vertices can be adjusted to a non-integer position either with mouse or resize, but not through keyboard input
Consistent availability of fine input (I would assume 2 decimals of accuracy should be enough)
Operating system & service pack: Win7 x64
Construct 2 version: r138
Edit: Might also be useful to use a modifier key like CTRL or ALT while moving the vertice with the mouse to have it move to only to integer placements as well.