In sketchup for web is it possible to select all dimensions in order to get them all on 1 layer.
Not in SketchUp Web Free. You’ll have to select them one by one to add them to your selection.
What you could do is to select all the geometry one by one to add them to your selection. Then (temporarily) hide that selection. Then window select around all dimensions to get them selected.
This could be a bit easier this way. Could be! On the other hand selecting “chunks” of all dimensions (to finally get them all) could also beneficial. To do the assign operation in stages.