How long should a meta description be?
Google cuts descriptions by display width, not characters: roughly 920 pixels on desktop, which is about 150 to 160 characters, and less on mobile. Wide letters like m and w use more room than narrow ones like i and l.
Google also rewrites many descriptions to match the search. A clear, specific description that matches the page's main search is the one most likely to be kept.
Questions
Do meta descriptions affect rankings?
Not directly. They affect whether people click, which is why they are worth writing.
Why does Google show different text?
It picks text from the page when it thinks that matches the search better. Make the description match the page's main topic.
Should every page have a unique description?
Yes. Duplicates across many pages are a common audit finding and are often ignored by Google.
Can I use emoji?
You can; Google may or may not show them. Test the length with them included.