Press CTRL, ALT and O together to show the grid then press and hold CTRL and a small options box will appear at the top of the screen enabling you to change size/opacity (you can also access the grid from the View menu/Loupe Overlay)
This doesn't work for the grid overlay that is part of the Transform functions. It simply changes the grid that is accessible as part of the View menu/Loupe Overlay. The one that is part of Transform >Rotate was fine but got turned into something so gray and semi transparent that it is no longer useful when you are rotating, etc. in the transform function. As I mentioned, it was fine for years but, with the new Lightroom Classic release, it got messed up. Hopefully someone will realize this and change it back or make it so the user can change it, as it is about useless as it sits.
I just checked it on LR Classic (on Windows 7) that I have and it still functions correctly.
I am able to set size and opacity using CRTL key. (And size only using toolbar slider)
Make sure that you have the Grid options set to "Always" in the Grid options on the toolbar.
It does not seem that the CTRL key works if it is set to "Auto"
You can change the grid with CTRL in ALWAYS but why do you need to see the grid when in the other panels. The problem is the changes made in ALWAYS do not carry over when you use AUTO. This is either a bug or a really poor design which would not be unusual in Lightroom..
Following up on another post with similar question it looks like the Grid opacity stays locked at ~20-30% when it is set to "Auto"
(Same behaviour as for LR5.71 and CC2015.12)
Adobe, please put the transform grid back the way it was or give us options. I agree it's not very usable currently. Makes me wonder how you test the interface. Seeing eye dog?
A number of people have complained about hard-to-see grid lines in LR 7 (Classic). Please add your me-too vote and details of your issue, including a full-resolution screenshot, to this bug report: Lightroom Classic crop overlay | Photoshop Family Customer Community . The more people who vote, the more likely Adobe might pay attention, and you'll be notified of any change in the issue's status.