Does this fetch my page to read its tags?
No. It renders from values you supply, either typed into the fields or pasted as raw <meta> HTML which it parses locally with DOMParser. Nothing is requested from your site. That means it cannot confirm the tags are actually deployed, only that the ones you gave it would look like this.
I fixed my og:image but the old one still shows when I share. Why?
The network cached it. Facebook, LinkedIn, and Slack all scrape a URL once and reuse the result for a long time. Fixing your HTML does not invalidate their cache. Run the URL through Facebook's Sharing Debugger and press Scrape Again, and use LinkedIn's Post Inspector, to force a fresh fetch.
What size should my og:image be?
1200 x 630 pixels, which is the 1.91:1 ratio every major network's large card uses. Keep it under about 5 MB, use an absolute https URL, and make sure it is reachable without a login. Anything under 200 x 200 is likely to be dropped, and a square image will be cropped unpredictably by different networks.
Do I need Twitter card tags as well as Open Graph?
Not usually. X falls back to Open Graph tags when the twitter: equivalents are missing, so og:title, og:description, and og:image cover it. The one tag worth adding explicitly is twitter:card set to summary_large_image, because without it X may render the small square card instead of the wide one.
How long should the title and description be?
Keep the title under about 60 characters and the description under about 155. Beyond that they get clipped, and each network clips at a slightly different point and on a different device width. The preview here clamps the description to two or three lines, which is a good approximation of the real thing.
Why do my previews look slightly different from the real card?
Because every network renders cards its own way and changes it without notice, and the same card looks different in the feed, in a message, and on mobile. These previews reproduce the layout and the truncation behaviour closely enough to catch a bad crop or a title that is too long, which is what they are for.