Second iteration:

Then as you approach a guide line, the width snaps to fill. I appreciate this is a bit awkward for wide and full widths as the layout will jump quite a bit, but it's hard to know just how impactful that will be without an interactive prototype (which I cannot make in Figma). Here's the closest approximation I can come up with:

If the above approach is too technically challenging, a smaller step could be to show snap-zones in a style similar to that of dropzones or the sibling inserter:

First iteration:

For blocks that have resize handles for width (e.g. Image), it would be nice if the block 'snapped' to predefined widths like wide and full.

If the above approach is too technically challenging, a smaller step could be to show snap-zones in a style similar to that of dropzones or the sibling inserter: