How long should a meta description be?
Google shows about 155 to 160 characters on desktop (roughly 920 pixels) and a bit less on mobile. Aim for 120 to 155 characters so the full message fits.
Too short and you waste space that could persuade people to click. Too long and the end is cut off, which often hides the call to action.
Writing one that gets clicks
Say what the page offers, why it's better and what to do next. Include the main search term: Google bolds the words people searched for, which draws the eye.
Every page should have its own description. Google may still write its own snippet if it thinks other text on the page answers the search better, but a good description is chosen most of the time.