Hit the O key to cycle through the crop/straighten overlays
Develop "Grid Overlay" can be set to show Never/Auto/Always by enabling it in the toolbar options (small triangle at the rightmost edge of the toolbar area)
Press and hold CTRL/Command while it is set to 'Always' and you can change the size and opacity, if set to 'Auto' then use the slider on the toolbar to change size only.
Note: this only changes this grid, crop overlay opacity is not affected.
Press R to get into crop mode and then as Bob said you can press the O key to cycle between the available crop overlays.
(Crop overlay opacity/size is not adjustable other than to change orientation by pressing X when the overlay is visible.
Nor can you add/remove any crop overlays.)